Just understand what it is you're looking for with this girl.
Don't get on her case when you discover that 21 yr old girls have different prioritiesand ambitions than a 31 year old man. It isn't her fault, it's just where she is at developmentally.
When you try to make them become "you" and where your at, that's when you will make the both of you miserable.
At 21 she probably still has some livin to do before she's ready to be a serious partner for anyone