I study but i still get bad grades?

I've reviewed many books on study. The best I know of are How to Double Your Child's Grades by Schwartz, Helping Your Teenage Student by Cohn and Complete Idiot's Guide to Study Skills.

For remembering course content, the first thing is to take good notes. This is not a skill we're born with. It takes a lot of time to learn doing it right. A source I like very much for how to do things - WikiHow. Read their article on how to take notes. It's great.

From my experience as a uni student, the two most important principles are organization of information and practice.

Write detailed outlines, with phrases, short sentences, and short paragraphs. Keep your lecture and reading notes separate but if you have a really hard test, you can take all your notes and combine them into one big outline. You can even - if it's REALLY hard - make your outline into a set of flash cards and memorize it.

Don't have papers all over the place. Have a filing cabinet or file box. When you don't have time to file things, keep them together with paper clips or folders, stacked neatly.

Don't cram at the last minute. Periodic review during the semester is smart. Another practice method is to occasionally rewrite your notes after class, which makes them better, with more details. Flash cards are useful in a number of ways. I found that I could make the cards work better by adding some mnemonic devices to them - a word or drawing that triggers a memory. The key to mnemonics is having things so weird you can't forget them.

Always study for an essay test and a very good way to do this is to write your own questions and answer them. This gives you a set of phrases and sentences you can use while you're taking the test and you have to hurry.

You've received some good suggestions to help with studying, including staying organized in your work space, taking notes (and for them to be more concise), and perhaps joining a study group (though, preferably meeting in a public place, such as the local public library, school library, or local book store).

When I was at community college, I would make "flash cards" out of "lined" index cards. That can be especially helpful with learning new terms and vocabulary, for instance. Just an fyi that the larger index cards (4x6 or 5x7) tends to be easier on the hand and eyes. Perhaps you may also consider making a mock-up test in the same format of your teacher, such as fill-in-the-blank, multiple choice, true/false, and/or short essay.

The how youa re studying is not working. You need to reevaluate your study skills. Take grades exams and see what you got wrong and what you got right, what kinds of questions were asked. Where did the correct answer come from, lecture, text book. Now adjust how you study for that class.
Talk to the teacher. You get a bad grade on an assignment, go talk it over with the teacher. Figure out where you are lacking when studying,
Study with other friends. Pick up on their good study habits,

I do not think anything is wrong with you. But the problem is you are thinking alot on getting good marks, shift your mind in learning the subject the way which is interesting. When I do not like anything topic in the subject I see youtube videos of it, which makes me to get to know & also to learn the subject.
Keeping yourself stick to the books are also not a good idea, you should do what you like too, if you like to play games do play for sometime, it will make you stress free & increase your productivity too.
Try to enjoy in your studies do not strain yourself

maybe you should seek tutoring