When I think about being a girl I become happy... VERY happy. I don't understand it, I don't know what it means. What's wrong with me?
There is literally nothing wrong with you. If you really do want to be a girl and this isn't something else going on (a therapist can help you determine that, BTW), it probably isn't going away and it certainly isn't your fault. It is generally known that hormonal exposure or lack of exposure to testosterone and estrogen in the womb is what causes gender identity to be different from biological sex. Since a lot of it is gestational, it is literally not your fault and has nothing to do with anything you have done. It just is, and you'll do yourself a lot of good by at least talking to somebody about it.
For the record, I am glad to see you talking about it on this forum at least. I didn't allow myself that luxury for far too long and it was very harmful. Please don't make the same mistakes as me and try to completely bury things, because it likely isn't possible. Talk to somebody if you are at all able to.
EDIT: For what it is worth, I had the exact same struggles that you are undergoing right now. Again, I will reiterate that you are free to PM me. No judgment zone, yo.