I actually went to a school counselor to figure out a good study method and these were the tips she gave me (and that I now use that have definitely reduced stress):
-On Sunday or Saturday, make an outline (very brief) of any sort of reading you have to do for that week in your classes. Don't try to learn the material, just look at it so you have an idea of what's going to be taught. The outline should just consist of key words and topics.
-Make a goal sheet on Sunday for the next week (for instance, "To be current in class material") that must be completed by a certain day, like Saturday or Friday. Then, make individual tasks that have to be completed to acheive that goal (like "review monday's lecture in anatomy", etc). Check it off when you've done it, and if you haven't done 80% of your week's tasks for that goal by your deadline, take away a reward you might have planned for yourself (like going out with friends on Friday night).
-Get one of those schedules where every hour is marked (like a day planner), go through and plan out your week. So, mark out hours for class, for exercise, for studying a specific subject, etc. That way you have an idea of what you need to do and when you've finished your day you have actual free time.
-Outline/make flash cards for your lectures after each lecture (so do it five days). This is a form of studying that will help reduce time cramming before exams.
-Review every day! I have this flash card program I use for stuff like microanatomy that I use every day. For other classes that aren't conducive to flash cards, I try to review outlines I've made for each lecture. At most it takes 30 minutes to an hour.
-Since you're like me and have trouble concentrating on material for a long time, first try out 25 minutes studying, 5 minutes break, 25 minutes studying, etc. Move up to 30 minutes studying, 10 minute break. At most, try to study 50 minutes, with a 10 minute break, but no more than that. I still have trouble making it past 2.5 hours, but if you remind yourself that you're going to have to do it later on anyway, it can help keep you on track.
Of course, this is all self-motivated so you have to make an agreement with yourself that you'll do it. You can also have someone who can act like a parent (maybe an advisor assigned to you?) that you check in with each week to make sure you are staying on top of your schedule. I use the counselor for that and it was helpful in the beginning. I don't have ADHD, but this schedule she gave me to use is also used for people with ADHD (granted, who may be on necessary medication), so I think it's doable for anyone.
Also, start being satisfied with grades that aren't necessarily A's! B's are just fine, too, and nothing to sneeze at. 😀