All of the suggestions above are good. I also might suggest that if you are struggling in a class (a math class, etc.) then you could go to a tutor (if you have not tried that already). Tutors are a great resource of help sometimes. I should know because I am a math tutor, and there was once this girl who failed her Statistics class. She had to take it over the next semester, and she came to me and the other tutors often so that we could help her - and she passed!
Also, try taking classes whose subjects you are good at. For me, that would be English or Math. Also, when you are reading your textbook for a particular class, try to put notes in the margins about the chapter to summarize important points. I found out that this saves a lot of study time later and helps me remember things better. Good luck with your studies!