How do I get motivation to do anything?

I’m on a throw away account just for my own personal reasons, but how am I supposed to…do anything?

I’m 23 now, and when I was 18/19 I would work out every day, eat well, and I was relatively happy. I get bodies change over time, I went from 180ish to 150 and now I’m the heaviest I’ve been at 200 and I can’t look at myself in the mirror most days. My libido has tanked significantly and though my partner says it’s fine, it doesn’t feel fine. I don’t feel like I do enough. I miss taking care of myself, but I can’t bring myself to do anything for more than a week before I give up. Is it something I’m doing wrong? I understand that I just have to “suck it up and do it” but I /can’t/. I don’t know why I can’t. How does anyone take care of themselves like a normal, functioning adult?