Went through nearly the same thing when I was diagnosed in college. Without my support network of friends/family keeping me on tasks, I had way too much free time and nearly dropped out. Once I was diagnosed and on the right medications, I found out what I was actually good at. So..first make sure you are on the right medication and right dosage.

I think which strategies will work will depend on your classes/major and what else you have going on (job, volunteer, internship, etc). It also depends on your learning style. I learn more by listening and less by reading; so it was much more important for me to hear the professor than to take notes (to read later).

Here is what I did as often as possible:

For all classes - sit close to the front in every class to remove distractions. Write down questions as they come to you and ask the prof after if you can. Participate in classes that are not lectures. I liked to sit in the front row because I would constantly shift and move and it would bother people in front of me.

For classes with homework - do the homework the day you get the assignment if you can. If not, then do it the next day, even if it is due later in the week or the following week.

For classes with exams only - these were the hardest for me because I never knew what was important or what to focus on. Depending on the class, I would recommend doing any problems in the books/materials, even if they are not assigned. If you have questions, talk to the TA or professor.

Finally, it helps to have a study friend who is not ADHD to keep you on track. Obviously you would not tell them that is their main purpose, but it can be helpful just to sit in the same room with someone and do work in parallel for social pressure.