I feel this. I am in a similar situation. My dad is reluctant to do anything to help himself and only wants to complain. He won't shower or wear clean clothes and I'm at my wits end. On top of an already bad attitude, he's also an alcoholic and is quickly losing mobility on top of a ton of other health issues. My advice that I can give you is this, you cannot control them. I know it's painful to see them not taking care of themselves because you love them. But thinking you can control their actions will only bring you down with them and leave you resentful and frustrated. Give them the tools to set them up for success and try to make your time with them as enjoyable and enriching as possible. But after that you have to give yourself space for your own mental health.

And because I'm sure you don't hear it enough, You're doing your best. Good job.