How do you get over the guilt of not being there when a loved one passes away?

I have been told by Hospice workers that many times a dying patient will wait until someone leaves the room to get a glass of water, something to eat, go to the bathroom or anything else. The patient will die then.
the Hospice Nurse told me that dying is a "personal" thing and if they want to be alone they will wait.
Your mom did exactly what she wanted to do.
If this has been 3 1/2 years you might want to talk to a Bereavement Counselor or find a Support Group.
You have done nothing to feel guilty about. And would your mom want you to feel this way for so long?

After 3 ½ years later you still regret not being at your mom's side when she died. Death doesn't come at anyone's choosing. I spent many hours at my wife's side during her last days, but wasn't there at the moment of death (also 3½ yrs ago). How would it have affected you if you were there? You say you're bothered by not being there. To what extent? Do you find it hard to concentrate, sleep, eat properly? If so, you may want to seek professional counseling. If not, reading some books on grief and grief recovery may help.

Can I tell you something please?

Everyone goes alone. It doesn't matter if there are a hundred people surrounding that person 24 hours a day and don't leave them alone for a moment. When it's our time, we all take that journey on our own and leave when it's time for us to go.
We come into this world alone and we leave it the same way.
You were most certainly "there" for your mother. You would not have written your post or be feeling bad about her passing if you weren't.
You know this and your mom does too.
How about showing yourself a little kindness? You didn't do anything wrong going home to take a shower and to get a few things done that needed doing. Your mom went to the next life when she was ready to go and she is at peace. God bless her and keep her.
It's okay for you to grieve and miss your mom. It's not okay for you to put guilt on yourself for leaving for a little while because that wasn't wrong.

I believe that when a person dies without their loved ones there, that that is exactly how they wanted it to go down. Your mom I can only guess, didn't want you to remember her taking her final breath. And of course no one dies alone. They not only have God/Jesus with them, but also their loved ones who have gone on before them, so please take comfort in that.
The fact that it has been over 3 years now, tells me that you would most likely benefit from some type of therapy or counseling, as your mom would certainly not want you feeling guilty about something you had absolutely no control over.
Life is short and it is precious, so do whatever you need to to get your joy back. God bless you.

It’s doubtful that your mother was awake and alert enough to know you weren’t there. Dying asleep in a warm bed, that’s the dream, right?

I know it’s hard to reconcile yourself to the fact you were doing self-care instead of her care, but had she been awake, she would have shooed you off, telling you to look after yourself. What matters most if you were there while she was ALIVE.

Forgive yourself. Your mother would want you to cut yourself a break.

I look at it from the vantage of a near death experience I had when I nearly drowned. It was the typical white light tunnel experience--filled with light and love. Because of that experience, I no longer fear death and view it differently than others do. Months before my father died, I told him I would not be at his hospital deathbed because I don't get along with one of my siblings. I didn't want bad vibes around him at that time and I knew I would not be of much comfort to him if I was there. I was with him the day he had to go to the hospital. I knew he was dying. I tried to comfort him the best I could as I knew I would not see him again. He knew it too. We had already made our peace for months before his death and said what we needed to say. I am the sibling who cared for him during his last year on earth. That was my long good-bye. I think that what I did is more loving and comforting than simply showing up at the deathbed as my other siblings did. I believe that nobody dies alone. Spirits come to greet and guide us. Take comfort in knowing that. When someone is ready to leave, they are transitioning. My father had been transitioning for weeks before his death and I knew he was ready to leave. I don't feel any guilt at not being with him in the hospital because we had closure before his death and I know he is in a better place.

I hope this will help you: I was with my mother much of the time in the several years before her passing, while she lived in assisted living and through some hospitalizations. On the day she was to pass, of course I didn't know it was THAT day, I was with her from noon until about 6 PM. She was resting OK, but it was clear she was in her final day or two. She was not alert, not speaking. I was emotionally and physically tired and, in my "denial," I said to her that I would return to the hospital to see her the next day with two of my brothers and my husband. Silly me, I thought she would be available for that gathering. I wanted her to be available. Ha! The Universe had other plans. Shortly after I got home that night, I got the call from the nurse.
I honestly believe my mother was actually waiting for me to leave. It was so soon after I left, that she passed away.
My way of dealing with the guilt was to tell myself I had intended to bring the brothers, etc., but Mom had other plans. That was her style, to "take control" and not let us see her less than strong if she could help it. And I knew she was in good hands. My father, who had passed 17 years before, "spoke" to me while I was still in the hospital room that day, telling me "they" would "take it from here." I had also sensed my grandparents and others close by in spirit while I was still in the hospital room with her. Through the grief and the guilt I felt for leaving her, in retrospect I know now she was truly not alone.
You were there with her when she needed you. You left a very good reason: self-care, so you could be strong again the next day for her. Try not to beat yourself up for this.

I was with my dad when he died and it really helped my perspective. His last day he showed no sign of knowing anyone was there. He was sleeping a deep sleep and couldn’t be disturbed. Though we were told he was hearing us, I’m not so sure. He was peaceful and it seemed to be very much a solo journey for him. Both my siblings missed it and I’m not bothered for them. Our hospice nurse told me the day before he died that in many ways he’d already gone and his body was just catching up. I hope you’ll realize that your mom was at peace and forgive yourself as you did no wrong. Wishing you peace

A simple changing of the wording you use when you talk to yourself may help.
I am firstly so sorry for your loss. But what you are feeling now is "grief", not guilt. You hoped to be there when your Mom passed. As a nurse I can assure you that patients often wait until relatives leave so they feel more free to pass; they often spend time trying to help the grief of their loved ones when they would rather be involved in the work of letting go.
The word "guilt" assumes that there is something you could have/would have been able to do about this loss. Grief recognizes that you have sustained a hard loss. If you need help with your grieving there are groups and Licensed Social Workers who are certified in special training for life transition work who can help you. May your good memories sustain you.

You say that you can't seem to get past that you weren't there for her. You WERE there for her. Is a person's last breath any more important than any other breath they take? Only in the mind of the survivor. What is important is your relationship with your mom and what you did for her when you could. I am not sure why there has been such a build-up in our minds about a person's last moments.
For some it may be helpful to be there. For some it may be traumatic to see a loved one pass. But I don't think your loved one is dwelling on it. If they are sleeping in death, they are not aware. If you think they are aware somewhere, I cannot imagine them dwelling on their last few moments on earth.
Whatever you did or didn't do, there is not reason to dwell on it. Forgive yourself as we should forgive ourselves and others.