My OCD around a topic has faded but the situation itself hasn't and still sucks.

My cat is extremely overweight and my mom won't listen to me when I say to put her on a diet. She just gets fed up that I won't stop talking about it, and ignored the vet when the vet gave her a website for diet advice. I wanted to see if I could take over feeding her but I don't know exactly how much food she really needs to lose weight and I worry I'll underfeed her, and also, my parents have a specific feeding schedule and would be angry if I messed it up. The problem is, I've loved my cat since I was a child and have never really experienced a conclusive loss in my life (the closest I got was nearly losing another cat as a child), so I don't know if I can handle it without me failing my classes. I also have ADHD and since in the past even the slightest emotional turmoil I felt made it almost impossible for me to do well in school, and all I got from my parents was anger and disappointment rather than any acknowledgement of my mental health problems, this just amplified my fear further. When I brought up how I was feeling they would just say that most people get bad feelings but they aren't careless enough to let those bad feelings cause actual problems.