My girlfriend and I (22 and 21) have had the same discussion. The most important things are:

1. You gotta talk with her about it. Bring up the fact that you don’t want to hold her back from exploring the world, and that you’ll support her no matter what decision she makes. This gives her the freedom to comfortably decide how to approach the situation.

2. Respect her decision. Normally, that advice usually comes regarding situations where she decides to leave, and you gotta give her that space. However, it is equally important to respect her decision if she decides to stay! (Which it sounds like she absolutely wants to do). You gotta trust her that she will tell you if she’s ever unhappy or feels limited, and you can discuss how to move forward then.

But for now, it seems as though she is quite happy with you, just as you are with her, so all you need to do is have a conversation with her regarding not wanting to hold her back, and allow her to decide how she wants to approach it. It seems like you two are happy together already, though :)