I completely agree with that, I get angry because I'm doing everything I can to keep her healthy and out of the hospital and she just doesn't want to do it. Mostly the drinking water. And when she doesn't do it, I get mad. Definitely it will take a lot of practice. I just always seem to forget about trying to manage my feelings when i'm in the middle of it going on.
To me, feeling mad isn't the problem. Anger is often a completely appropriate feeling, especially when someone else is doing something frustrating, harmful, or challenging. It's how we manage the feeling that is the key. If you feel mad and then argue with your mom, it's possible that you are not actually managing your anger, and instead are trying to manage her. I have done this myself many times, hoping that I could convince another person to do what I wanted, so that my anger would go away. As you have seen, that doesn't always work, and it often leaves us feeling even more angry because it appears that the other person does not care about our feelings.
When others refuse to cooperate with my expectations, I have a choice. I can keep asking them to follow my plan, which they may or may not do, or I can accept that I do not have any control, and then decide what my next step will be. Any emotion I have then becomes mine to manage. If I expect someone else to do something to improve my emotional state, I am effectively giving away my power to feel better to someone else.
I have found that the most productive thing to do when I am angry is to allow myself to feel the feelings and then figure out what action I can take to productively manage the feelings (walk away, journal, sing, pray, etc.) I also take action to avoid the problem in the future. So if my mom were refusing to shower, for example, I might tell her that I was going to hire an aide and then do it.
She still gets to choose her actions, and I get to choose mine. As a parent of now-adult children, I learned a long time ago that people have free will, so all we can really do is decide how we will respond when they act, especially when they do something that disappoints us, angers us, frustrates us, or irritates us.
It takes some practice, but adopting this mindset has created some healthy space for people in my life to make their own choices, and then for me to feel safe with mine.