It's probably not you, man. Sounds like her mother has a hard time letting her 'baby' grow up. For example, I've been married, divorced, and in combat, but when I'm home I'm 16.
It's definitely going to be tough, and might not even be worth it, but you're going to have to either prove yourself to them by being an adult, or ignore them and go wild. Whichever way you go, make the best of it. You only live once, remember, and most people here would give up anything they owned to be a teen again. Christ, I'm only 23 and I wish I was 15 :
Also, do things for YOU, not for her parents.