Sounds like you're pretty stressed out; but refrain from using stimulants, there is no magic drug that will make you smarter.
Anyhow, I finished college with a 3.9 GPA and here's the technique I used, and you need to buy lots of index cards (which are quite cheap thankfully)
Every time you go to class, take good lecture notes. As soon as you're out of class, get out your index cards.
Take each piece of lecture info and turn it into a test question by putting a question on one side and the answer on the back. For instance, if you have a lecture note that says "George Washington was the first president" then on the front of the card write "Who was the first president?" and on the back write "George Washington", but I'm sure you get the point. Do this for EVERY relevant lecture note you've taken for that class day.
Next, go through the cards one at a time. Read the question, then see if you can answer it before flipping it over. Any answers you get right before flipping over can go in one pile, and any you get wrong should go in a different pile. Then go back through the ones you didn't know until you get them right without looking. Repeat until you can answer every question before you flip over the card.
Then do the same thing after your next class, but add the cards to the ones you've already made and go through the whole stack. Keep repeating this process until you have your big test.
I know it sounds like a lot of work, but it gives you an objective and a goal rather than just winging it.
Good luck to you!