FWIW, I'm nearly 50, and was diagnosed when I was in my very late 40s. Aspergers wasn't even a potential diagnosis until after I'd graduated from high school, and it's now called Autism Spectrum Disorder. My parents were very concerned about me from a very young age; they took me to a specialty learning psychologist when I was about 6 (I still remember that he had neat lava lamps in his waiting area), and he was stumped. He said that it looked a lot like ADHD to him, but he was pretty sure that it wasn't ADHD.
I've gone through things like this all my life, where I'll get extremely focused on one thing for a while, and then suddenly lose all interest, and I can't force myself to care anymore. Sometimes I'll hit some kind of roadblock on something, and my the time I'll have cleared the roadblock, my ability to care is gone. I guess it's true to say that I don't have discipline, but it's true in the same way that a man with no arms has shitty grip strength; that 'muscle' just doesn't seem to exist for me.
That's where I am right now with hitting the gym. I'm trying to get back into it the way I was from 2013 - 2020. It's hard; the way I felt before just isn't there, and finding the motivation ever day is a challenge. Even things that should be habits, like showering every day, or brushing my teeth, take constant focus to do. Cutting firewood? Sure, I can do that until my back give out, I can no longer lift the chainsaw above my shoulders, and I can't grip the maul. But getting the motivation up to get started, even though I know that I'm going to need that firewood in nine months, is so brutally hard, and it seems like most people don't struggle with that.
In my experience, my parents forcing me to do things once I lost interest in them--or if I had no real interest in the first place--wasn't helpful. It didn't make me feel more disciplined, it just made me feel like a failure because I didn't have the enthusiasm that everyone else had.
Oh, and after 3 years of Little League, I can still say that I hate baseball.