I want my girlfriend to only be mine?
Thats not too unusual. Why do you need to clarify that you want her to be only yours?
She's my partner, and I should be the only one she wants.
It's fine to want an exclusive relationship, but don't become overbearing. It usually just pushes them away. Let them be themselves.
I'm not going to look down on her for having her own friends.
Thats good to hear. Just because you care for someone doesn't mean you shouldn't let them live their own lives. I'm sure she'll appreciate having her freedom while still coming home to you.