Part of loving somebody is allowing them to grow and to reach their own potential as an individual. To stand in the way of that is also denying the relationship the opportunity of growing as well.
It is a difficult thing to do when you love someone to let go and be confident enough in the relationship to withstand it.
The way I see it there is more benefit in supporting your GF's growth and your relationship's overall success than there is of denying her something that she needs to experience as a young adult.
I have a favorite saying that may help you through all of this. It says...
If you love something set it free...
If it comes back to you it's yours forever.
If it doesn't, it never was.
Hope that this helps you through doing something that the heart sometimes finds difficult to do.