As
@Razul said, motivation shouldn't be your goal.
If your body clearly doesn't want to do something, then why do it? You wouldn't need motivation for something you're clearly supposed to be doing.
I have ADHD aswell, so i can drop some examples.
When i started off dieting, back in 2022, i hated it. I spent my whole life eating junk food so when i forced myself to eat well it just didn't work. At first it led to a lot of starvation and then just looped back to eating junk food.
Same was with the gym, it was fine for the first month but then felt like a chore.
So i thought.. why force myself to do things i dislike? And instead chased things that i liked.
I started eating food that wasn't junk but was actually good and i felt good eating them. Instead of lean chicken i ate good juicy red meat, instead of plain water i drank fruit juices and milk, when i wanted sugar i ate delicious exotic fruits. Also, instead of the gym, i went for long walks in the forest, maybe a little bit of calisthenics here and there. Now that's my daily life and I'm enjoying myself a lot more than ever. I'm actually happy with my life and with myself.
When you're enjoying yourself you don't need motivation, because you WANT to do it.
So if you need motivation to do something, it's most likely something that you shouldn't naturally do in the first place.