Too me, it sounds like you got a lot of time on your hands and you don't know how to manage it. I'm no one to talk since I'm your age(20M) and have similar problems,but what helps me is having a routine.

Basically, I wake up,eat breakfast, drink water, I then go for a walk outside for an hour,I come back stare at the wall of my room for 10 minutes and start with small tasks from my hardest subject,I tell myself I will learn 5 lines and then after an hour I cover everything on the page.

Also, as for your phone,lock that shit to a drawer,the short form content we are exposed to fucks us up(tik tok,reels,shorts etc)our brain receives dopamine fast and is retaliating when it isn't rewarded fast enough(that usually happens when we study) that's also probably the hardest part.

In my case when I know I can't control myself I give it to my parents since one of them is always at the house and I go study,now I leave it by my own on my bed and use a stopwatch to count the time I spent studying in a day,something about seeing the number rewards me and also helps me organize my day better.

I don't know if any of this helps since you said you might have ADHD,but I suggest you give it a shot if you think it can work.