I don't wanna end up being a "stranger" with my best friend I wanna keep being BFF with her even after we graduate and all
As you grow up you may hopefully learn:
1. That these things cannot be forced to happen the way we want them.
2. They will happen how they will.
3. That it is better that way.
All you can do is continue to be the good friend to your BFF that you are now. If you start being controlling, needy, clinging, whiny, etc, you may drive her away. Of course you may also get a boyfriend of your own, unless there is something you aren't sharing that (a) makes this unlikely and (b) explains your anxiety about your BFF getting a boyfriend?