My wife says all women are fickle and with her being young there might be confusion, but what my wife doesn't quite accept is that to them, we're dinosaurs.
Does your wife see something that you don't? Girls do stupid shit all the time.
Is the GF insecure or have low self-esteem? If so the odds are she will fuck up a good thing.
Where is her dad? IMHO nothing is more important to a girls mental state that a healthy relationship with her dad, i.e. a "Daddy's Girl" If she didn't grow up with a man she could depend on and love, she will never be able to trust any man. Even if the right man comes along, she will do something to fuck it up.
And dinosaur? WTF is that about? Bro, we are probably around the same age, as often as I get hit on by girls between 17-19, you would be out slaying it if you didn't have a woman!