My mom was just diagnosed with 3 months to live and I don't know how to handle it without breaking down in front of her.

      Jay, I am sorry that your mother has been given this diagnosis. I would definitely not look at the time as written in stone. I have known many people to live far beyond what the doctor predicted. You have been given some wonderful advice in the previous posts. Being strong for her means that you are honest about your feelings without burdening your mother with them and making sure that her days have as much meaning as possible and as much comfort as you can give her.
      With that said, I will address your references to God. My first thought was to let someone else do this, but I decided to respond in the hope that I could help.
      I can only speak from a Christian perspective. First of all, it is OK to be angry at God. He is big enough. He can take it. Rail away. Shake your fist if you want to. The God of the universe understands our human emotions and frailties.
      I am going to look at this through the eyes of your mother since you said “her God”. If she is a Christian, whether she followed Jesus all her life or just in the last few moments, she is looking at eternal life with God after death. We were never promised eternal life on earth, or for that matter, a life free from turmoil. We don’t serve God to be rewarded in this life or even the next, so God didn’t renege on a deal though your mother served Him all her life. She served God out of joy and gratitude for the forgiveness of sins, grace and mercy found in this life and eternal life to come. Her serving God was not a bargain, it was a natural response to being born of the Spirit.
      Again, through your mother’s eyes. She will be sad to leave you, she will worry about leaving you, she might be afraid of the physical act of dying, but rest assured that your mother’s eternal life with God began the moment she believed and put her trust in Jesus. 
     Be with your mother one day at a time. Take your eyes off the timetable. For today, comfort her, laugh with her, brush her hair, talk to her, pray with her, (or if you can’t, ask her to pray for you out loud if possible). For today, let her tell you stories of her past, or you can play “Remember when?”. Let her know by your actions and words that you are going to be OK - for today. For today, your mother is alive. I pray that these final days with her be a time of growing peace for you.

Jay, I'm sorry to learn of this sad diagnosis. Perhaps you can manage to view it another way, i.e., as spending quality time with her, easing her pain and anxiety, and being the best daughter you can when she needs you the most. And that last point is important - this is probably the time when she will rely on you more than at any other time.

I won't address the religious issue as I'm not religious. Perhaps your rabbi, priest or pastor can help you with this aspect.

As many years as I cared for my father before he passed, I still think of things I want to say to him. And I also wish I'd just set aside the time to just do the things we both enjoyed. Those times we did enjoy those activities are more precious than all the doctor appointments hospital visits, and all the medical activities.

Eventually I'll forget about the trauma and nuisance of the medical visits, but I'll remember when we planted morning glories along our back porch, and came out every morning to count the number of blooms. I'll remember how often we checked the pumpkins in our garden, anxious for the harvest so we could carve the pumpkins for Halloween. I'll remember when we went for walks at the local state parks. I'll remember the times we just sat and talked about life. And I'll remember the times when we worked on projects together and he explained how to use various woodworking tools.

Perhaps you can think of that as your mother proceeds on her final journey. What do you want her to remember you by, and what do you want to remember most of her? What do you want to share with her about your life together?

If you have to cry, make an excuse to go to the restroom and cry in private so that you can continue to be strong for her, and not concern her with your own grief. It might only make her sad that she'll be leaving you. Since you're religious, perhaps you can take pleasure that you're honoring her in her time of need.

Learn as much as you can about her illness(es) and act on her behalf with the care staff to ensure she receives a high quality of care.

Bring her favorite music and play it for her to help her relax; decorate her room with flowers, photos of the family, things to help calm and soothe her.

I wish you peace and solace on this challenging journey.

I suggest you take photos of old times and go through them with her. I respect you for not wanting to break down in her presence, but will say I consider crying to not be a sign of weakness. God can handle your anger.

I will add a comment about my experience. I do not do so to suggest you should do or feel likewise, but just to share....My wife was profoundly handicapped from a massive stroke and in many ways from 2005 until she died in 2017. A month in to her illness I realized she would never get better....I felt lost. I asked God to give me a servant's heart. He did. Life was not easy for me; it was full of suffering for her. She was also full of joy at all times. I can't explain why. I look back upon those 12 years ass the most satisfying of my life.

Grace + Peace,

Bob

Jay, I am so sorry you are dealing with this.

I agree with GardenArtist that you don't want to spend all your time with your mother in a state of grief and sadness. But if I had a diagnosis of a short life expectancy I think I would be very hurt if everyone just bucks up and takes it in stride in front of me. Gee, isn't anybody broken up over this? Don't spend the next three months wringing your hands and using your limited time with your mother being miserable. But I don't think you have to totally hide your feelings.

GardenArtist has given you some good advice on focusing on what is pleasant and meaningful.

Jay, great, caring and comforting advice above.

Your profile says your mom had a stroke/heart issues. Is her life-limiting diagnosis based on those issues, or is something else going on?

Have mom's doctor's talked to her about her diagnosis? Is she willing to talk about palliative care or hospice? Those services can be of great comfort to both the patient and their loved ones.

I'm so sorry that you are going through this pain with your mom. Be gentle with yourself and let us know how things are going with your mom, and with you.

Jay, I’m very sorry you’re dealing with this. You’ve received some wonderful advice already. My mom passed away after a horrible turn of rotten health events, it was the worst thing I’ve ever witnessed. She was a faithful Christian woman and I have certainly spent a lot of time wondering why she had such a hard road in the final years and sad death. Ultimately I just have to believe that God had a plan in it all that I’m not capable of understanding, I do know that many people were positively influenced by my mother’s last years, and awful as it was it did serve a purpose. I cried many a tear after leaving my mom’s side, but tried to make the time with her positive and enjoyable to us both. I told her a new joke every time I visited. She enjoyed a good laugh even through horrible circumstances. I held her hand, I took her outdoors to see flowers, I talked to her about everything and nothing. I hope you can find your way to not spend this valuable time not being angry, but focus on your mom and what will help you both. I wish you peace and blessing, it’s so hard, but I so want you to have memories of time well spent

I am sorry for your grief. I truly can identify and when this happened with my loving Mother and myself it was an amazing time. She said things to me only a dying person could know. A distant old friend arrived in a "wheelchair" for a visit (she came a long way to see my Mom) when all others thought she would come through yet another hard call by the Doctor's. My Mother left us with the most loving hospice nurse only 10 days later. Others had wished they had come to see her. You are so blessed to be so close to her, and she knows this I am certain. Waste not a moment - listen to her words, they will "pop-up" in situations later in your life. Be in the now, with her. Most important - tell her how much you love her and don't forget to say thank you. I screamed so loud as my Mom passed at home with me it echoed down the canal...but I shall never forget those last few weeks of listening to her stories. You are blessed, the chosen one here. TY for sharing this with us all...stay strong. God Bless.

I know that a lot of people think that THEY are the ones who have to deliver "the bad news" to their loved ones. I'm hoping JayC doesn't think that it's his job, and that's he's distraught over the general discussion of EOL with his mom.

Doctors are the folks who should be doing this; they have the training and can answer the hard questions that inevitably arise during discussions of end of life issues.

Jay....
If your Mom is not on Hospice I strongly suggest that you call a Hospice in your area.
Not only will they help your Mom they will help you and the rest of the family.
I, and every one on this site will agree that this is not an easy thing to go through, watching a loved one die.
All I can tell you from what I learned caring for my Husband is this. I developed an insight that I never had. There was a bond between us that seemed deeper than it was when he was well. (He had been diagnosed with Alzheimer's and I think he had Vascular Dementia as well) The fact that he was non verbal for probably the last 3 years of his life did not seem to matter to either of us, we communicated pretty well I think.
All you can do is do what we should all do every day.
Appreciate those around you.
Forgive quickly. And this also includes forgiving your self. (You make the best decisions you can at the time don't spend time with regrets)
Accept help when it is offered.  And do not hesitate to ask for help. (friends want to help they just don't know what to do so give them a task)
Give hugs, hold hands.
Learn to love life to the fullest.
When you go to bed at night and you put your head on the pillow and close your eyes if you can honestly say that you did the best that you could that day...that is all that anyone can do.

On a practical level, if you feel that crying in front of her would upset her: you could write her a letter you might or might not read parts to her later, to get some of it on paper so you don't feel as overwhelmed. It will release some of it; trust the many who say this helps.
I also went to the grief section of my local library. Others are helped greatly by a caregiver's group.

But letting her know you're sad (and appreciate all she has done and meant to you) *is* appropriate.
Otherwise the stiff upper lip can be more depressing to both of you because it may not give her the freedom to acknowledge what is really happening.
When my mother was fading I remember reading a comforting book advising that it can help to say a sincere version of:

1.Thank you (I elaborated on things she'd provided and acknowledged struggles she'd had worked to overcome to give us what we needed, etc.)

2.I'm sorry (in my case that I was a difficult teenager or didn't stay in touch with you as often as I could have)

3.We'll be okay (grandchild, family situation she worried about etc.)

And then *much* later in the process something like
it's okay to go

Please give us an update and take care of yourself!