I'm upset that my gf wants to move across the country with her friend
If your girlfriend went, what would you do?
I'd slowly cut her out of my life, without her noticing it.
Most people don't move far away from a partner for a friend, but you also can't really tell her not to. If you plan to cut her out of your life, you might as well break up before she moves.
I wanted to go out of town for a short vacation. I'd wait until my vacation ended and break up with her then.
I understand that you are upset about it, long distance relationships are hard but you cannot stop her from moving. It would be good to end things as soon as possible so you can move on.