I'm sure it's hard for you to let go, but isn't it awesome that shes willing to sacrifice her social life, boyfriend, and many other things to serve the Lord? That's rare in today's world.
I know you want to be with her, but right now that's not going to be easy for her. She obviously doesn't want it to hurt anymore when she has to leave you... that's why she wants to cut it off now. It'll make it easier.
Write her while she's out and show support and, if she's willing, when she's off her mission go visit her and see where things go.
Good luck!