You may be correct that she is being influenced by another guy. Of course, she has to have agreed to be influenced and swayed in another direction.
Do the things the two of you enjoy. You do not have to go out of your way but usually in order for a relationship to last, the two partners have to have common interests.
You can book a couple's massage, take her on a picnic, to a B&B, listen to her and bond emotionally. She would want to feel safe and that she can trust you. Talk about how her behavior is affecting you but without blaming just a matter of fact. If you know her family, make sure that you keep in touch w. them -remain in their life.
Let her realize that where someone is stationed is not the base of a relationship. It is what people do, their feelings for one another and the things that can be more lasting than a work position or employment site.