I can tell you're upset, I would be too. You may feel grown up, but at 15 your brain hasn't caught up with your body. Don't feel bad about it, it's just how God made you.
First off, you mention that you're not the best behaved kid in the world. Again, you're probably still a decent young man, but you recognise the fact that there are things that you could work on. Look at it from your girlfriends father's point of view. He thinks just as highly of his daughter as you do, and he wants nothing but the best. No one's asking you to be the pope, but make some changes to your behaviour if you really want to be in a relationship with this young woman.
But, even if you become Mr.Wonderful, it doesn't work out, God's got another plan for your love life. You'll be hurting, and it might take a while, but sooner or later you will find the perfect lady for you.
Good luck!