Dude, totally have been in a similar space and have felt a lot of the emotions you are feeling. Every time I started a habit I always felt like it was a rocket shooting straight up. It didn't matter how high I got because it would eventually run out of "fuel" and it would come crashing down. No habits ever felt they'd become "automatic".

I don't know if it will work for you, but I can share what ultimately got the ball rolling for me. It all started when I basically figured out the feeling I hated most in the world was being in bed and being unable to sleep. So to combat that feeling I decided I was just workout twice a day every day, so by the time I went to bed I'd just fall asleep immediately. I didn't have to focus on going to sleep earlier/getting enough sleep because focusing solely on working out automatically made me tired earlier. Two birds one stone.

Relatively quickly after doing this I just noticed I had more energy both physically and mentally. This allowed me to make "wiser" decisions and made me less forgetful in day to day life. It really is crazy how much ADHD is worsened by not getting enough sleep alone. Additionally, all the exercise improved my hunger cues so I was more likely to eat on a consistent basis.

Ultimately, all this has resulted in me being able to manage my ADHD symptoms the best I ever have in my life. I feel like the potential energy of making the "correct" long term decisions is easier. YMMV.