what should i do to make it better?

So I pretty much screw up everything i do. I do something great and then go right back against that and screw it up. I try to be the best for my girlfriend but i always end up messing it all up. I love spending time with her, I love being with her, I love talking to her, and most important i just love her. She is the one thing that makes me happy no matter what, all the time. I dont want to lose that feeling that I get from being with her but it seems like i am forcing so much on her. I think I am starting to cause her more stress than school usually does. I just wish that i could quit doing things wrong. I know she thinks we should break up, but i dont know why she hasnt yet. I guess i am just getting lucky to have her as my girlfriend. She means so much to me and i have already lost her once. That feeling i got when we weren't together was almost unbearing and i have no idea how to explain it. Any advice would be greatly appreciated.