It must be very frustrating to go from one emotion (love) to an extreme opposite (feeling annoyed) so often. If you want to have a healthy relationship with someone, it is imperative you have a stable sense of emotion. You should talk to your girlfriend about how you feel and get feedback from her. You should also talk to someone, such as a school counsellor, to try and get to the heart of the matter. Talking to a responsible someone will provide you with a fresh and possibly insightful perspective on this matter. The situation won't get better though, unless you actively try to make it better.
Good luck.