Accept yourself the way you are and try to do the best you can of your possibilities. Stop thinking negative thoughts. Force yourself to think of what you want to achieve in the future instead of putting yourself down. Nobody will think of your extra years in school when you are grown up. I have met many people who did bad in school and extremely well in life. What interests do you have? What kind of occupation do you want to have? Try to eat better to improve your health.
Read more about healthy food.
Thinking positively.
Thinking constructively.