Hi,
I don't understand why you have to break-up with your girlfriend because of a career. Are you assigned far from where your girlfriend lives? Why do you have to break-up? Can she not move and follow you where you are going to work?
Breaking up with her means that your career is more important than her. Or perhaps you don't really love her.
If you love her very much, maybe you need to reconsider. If you are compatible, get along so well and have been together for a long while, perhaps you can both talk this over and agree to relocate together. Jobs are easy to find, but a good woman you are compatible with is not easy to find.
Take care and please be open with her, perhaps she can make some suggestions which both of you can agree upon. Overall, people are more important than jobs.