I think the biggest issue is how each of you see the relationship. Has see had some horrible break ups or something that would cause this commitment resistance. I'm thinking poly is attractive to her because of this not the other way around. If you after all the time, effort, tears ....don't have boyfriend status that says something. I get it now. How does she label the other relationship or guy..... is he a guy I "see" at events or FWB or fuck buddy or is he a long distance boyfriend.
About the other guy: one could argue if he had a huge trust fund he'd have the perfect life. As a guy I to think its about him getting laid....and I have no problem with it ....god bless and have fun ...that being said the CDC may have his picture up on a wall somewhere too.
I'm confused about her reaction to your suggestion of dating other women...it sort of implies her feelings are stronger. If you're just the guy she's seeing you would think it would be no big deal or even welcomed to get you on board so to speak. I'd keep your dance card open.... go out and "see" some others then talk it may help put things in prospective.
I don't know what you need to do mentally to help you before deployment but I wouldn't count on her for providing it for you. I think you need to down grade how you saw things and where you saw it developing.... she becomes one of the girl(s) I'm seeing.....something like that if that's possible.
I bet getting shot at is easier than this shit ...
Good luck and be safe D