How to stop harboring anger and resentment?

Try to find outside employment, even if every penny of your wages go to paying for care it may be worth it to get a break from it all.

In circumstances this difficult, I go for alcohol and sleeping tablets. Short term. That's the truth, not a jest.

So what exactly would you find helpful? If you are looking for dietary supplements to help with the physical toll that living in a stressful situation can cause, beets or beetroot powder are supposed to help with high blood pressure. Not sure of medical studies that would back that up but you can do research on it. Using a Tibetan singing bowl for meditation helps a friend of mine focus and calm. Certain herbal tea are supposed to help with stress but be careful with them If you are on any medication .
If you research ways to decrease stress and anger there are lots of options to consider. Hopefully you can find something to help for those peak stress times. To truly address the problem you are going to have to address the issue with your boyfriend one on one or with a counselor. A big issue to consider working thru is if you could come to forgiveness with your boyfriend after so many years of living with this degree of stress. You
have been given excellent advice from some wise women in how to deal with the big picture problem. If you are just wanting to get through what you are hoping is a time limited issue I hope you find what works for you.

https://www.psychologytoday.com/us/blog/anger-in-the-age-entitlement/202108/the-dynamics-anger-and-resentment

Here you go.
Read this article and then come back and read your responses to your post and you will feel like a textbook case for this doctor.

Notice that “blame” leads to anger and resentment.

Here is this doctor’s suggestion.

“The Way Out: Condition New Responses
In close relationships, frequent anger and resentment are a tragic substitution of power for value. Instead of doing something that makes them feel more valuable when they feel devalued, angry, resentful people automatically opt for blame and the temporary power of adrenaline, which will never make them feel more valuable.”

If you think diet and meditation might help you become less resentful and angry, by all means try both.

I’m curious? What would you like to be different with your situation? What would you like your life to look like in a year?

If, in a year, you hope to be enjoying a resentment free relationship with your boyfriend while he cares for his mom, and maybe even developing a relationship with her yourself, then that’s a noble goal and working on yourself with that in mind is admirable.

But if, in a year, you hope mom is out of the picture and your boyfriend is joyfully focusing only on you, diet and meditation are probably not going achieve that.

Anger and resentment towards a poor sick 91 year old woman? You made an unwise choice moving into this mess. Is BF caring for his mom or are you doing all the work? If you are, are you being paid or are you being taken advantage of? Who is paying the bills?

I suggest moving out. It is drastic but if it is affecting you this way, you need to make a change. Find a job and move, you are not married and can just pick up and leave.

Not sure what you are expecting long term from this relationship but from the outside it seems like this relationship is going nowhere.

I cannot fathom how this poor lady deserves your anger or resentment. How you taken a minute to consider how it might feel to lose your mental and physical capabilities and be completely dependent or others for every minute of your day, every bit of your care? This could be you, it could be any of us. Any suggestions? Decide to either be a help and take some of the huge load off as a caregiver or decide to cut bait and leave this man to care for his mother as he has chosen. Either way, he’s decided what he’s going to do. Your health will improve when you decide for yourself

Diet change?
Medications?
How about a boyfriend change, as well. I think that would be most effective.
You are angry at the poor MOTHER of this boyfriend?
I can't imagine how it is her fault at all. She is and has been ill.
It's the boyfriend who has treated you as slave labor.

If you're angry at someone I am sad to tell you that you are most likely angry at YOURSELF, and with very good reason. You have made very poor decisions for your own well being.

You will either now take care of yourself and leave the situation or you won't.
You're an adult and have free will to make your own choices. Only you bear the awful weight and loss of the choices you have made, and perhaps will continue to make. No one else is harmed; that's the good news.

So, it's up to you.
It always was up to you.
No one else. Just you.

Have you considered just walking away from it all and living your life the way you want, without a man who obviously puts his mom before you and your relationship, and who obviously doesn't care about what this is doing to you mentally and physically?
I mean...surely you realize that you deserve better right?
So that is my suggestion to you to put yourself first, know you deserve better, and pack your bags and leave for the life you deserve.