Sometimes you can study many hours but it doesn't do much if you're studying ineffectively. Have you tried studying in a place away from sights and sounds that are distractive, like the library, for example?
I noticed over the years that students who do poorly lack a good background in Chemistry. Before I took the G-chem I took Intro to chemistry. It was a review class, and I did so poorly. I was a studyaholic like you and I studied and studied. No matter how much I studied I got a C on the quizzes and hence pass the class with a C. 🙁 I can kind of relate to you, I know how it feels. After the quarter was over I did nothing but review intro the chem for Christmas break for 2 whole weeks until I knew the material inside out, forwards and backwards. I got into G-chem and kicked ass! After that I never ever got lower than an A- in any of my chemistry classes because of the background, and I've taken all my g-chem and 2 out of three classes of o-chem. If you feel that you are doing poorly in g-chem you should review your notes again before the midterm you just took, or maybe go back and take intro to chem (if this is possible). All chemistry material builds on itself, this is why background knowledge is extremely important.
Assuming that you studied so much and did so poorly may reflect the fact that you studied the wrong material. Look at the exam questions again. Did you "reflect" the exam during your studies?, i.e. did you study the material that was on the exam? If not then you have this problem. Sometimes professors give hints on what is going to be on the exam. You might think that this is trivial, but it isn't. Mark down the hint, and study in detail more on the material related to that hint. Practice midterms are excellent for giving you a good idea on what is going to be on the midterm. Looking at the concepts of the practice midterm you can make certain guesses what concepts the professor is likely to put on the exam. Make sure you know the lecture material well. In my experience with chemistry, lecture material has ALWAYS been more reflected on the exams than the book material.
Another thing I noticed about struggling chemistry students is that they don't go to lecture. They stay at home and read the book. THIS IS VERY BAD, DONT DO THIS!! Lecture material is extremely important, and should have more priority in being studied than the book.
Are you taking good notes? Students who struggle with chemistry seem to have this problem, especially in organic chemistry. They seem unable to write down important things like the properties of a reaction, or are unable to keep up with the professor. If so, you need to develop good notetaking skills, or you may need to keep yourself concentrated on the topics in lecture. See if you can keep yourself focused on what the professor is saying for an hour without having your mind drift away. I know this is hard because chemistry is extremely boring. Like I said before lecture is extremely important.
Finally, my general advice to you is... Try everything, try anything, you never know whats the successful way for you until you try.
I hope I was helpful.......
Good luck on future studies in Chemistry....