It sounds like you also may have attention deficit syndrome. You should see your doctor . He may want to have you tested and see if ritalin would help. It is the drug of choice to help with ones concentration. One tip is stay away from caffeine, as it will only make you race more. Drink decaffinated tea or coffee if you must.
see attached link.http://www.webmd.com/add-adhd/default.htm
If you are fallings asleep because your unable to concentrate I suggest you follow this link, It gives great tips especially for studying.You should also speak to the teacher regarding your problem before you even start the class as she may have some ideas. If you are unable to focus because of tiredness, you might want to seek a sleep center to have your sleep monitored in case its a sleep disorder. Because you mentioned you fall asleep after reading just 3 pages and that maybe a symptom of narcolepsy. However if you are falling asleep because of the adhd or depression. then I suggest you try the tips from the link, as speak to your md. If it is so uncontrollable you mayneed the ritalin, however perhaps with those tips you might be able to get it under control without medication. Here is the link