You don't give her ultimatums and you don't contact her. Don't grovel and get all macho either, it is not a good look and it does not work. Leave her alone.
I know you are hurting but the thing is, most relationships that start in the teen years and then go through time of separation because of college or military or whatever don't survive.
Respect that she wants to be away, let her be and you move on as well. It will get better.