I am sorry that you and your partner are hurting - I can understand how you feel. What she did definitely was not good, considerate or right at all.
You came here asking for unbiased opinions and in the hopes of giving you something to think about... here are a few of my thoughts about where your end of the bargain might be worth looking at:
(These are just ideas that spring to mind - I am definitely not placing the blame on you in the slightest).
Thinking back, were your relationship guidelines and structure discussed, or were they unwittingly dictated?
For example, I've said before to secondary partners that if they sleep with someone else, I would like them to tell me, for reasons of trust and sexual health. However, what I didn't do until recently is say "I would want to know about it - how does this realistically fit with what you want? What can you offer me and what do you expect? What arrangement would be honest to who we both are as people in this relationship?"
All too often, without even realising it, we can get into a mindset where the primary couple call the shots. This can be shown in the simplest, most subtle of ways and sometimes it passes us by.
The other thing I'm wondering is whether or not she was realistically aware that she was a secondary - and if she was, what this meant for her. It struck me that during the seven months of dating, much of your time has been apart? I do know that meaningful relationships can be maintained from afar - my primary lives in an entirely different country to me and we spend half of our year apart. But I'm wondering if she wasn't sure of, or wasn't read for, the level of commitment expected - especially with her being in the Navy and then being at School?
It seems that you told her to let you know if she met someone who might change your relationship with her - rather than just letting you know, period? Perhaps this caused some confusion in her mind?
All of this being said, what she did on NYE was utterly rubbish. Either she wanted everything and wanted to lie to get it, or she wasn't being honest with you about what she actually wanted from you and perhaps felt caged; her only option to do something extraordinarily awful, for a way out.
In terms of introducing partners to other partners, then finding that they disappear on you - it could be a variety of things. It could be pure bad luck. It could be that you ask too much of your secondaries and they seek something more casual with the new people they meet. It could be that you don't give enough to your secondaries, or that they need more than you can give.
The primary/secondary debate is a big one - but I genuinely believe that some people *are* happier in secondary roles. Sometimes it's intimacy issues, wanting general freedom, not having time, being happy with the status-quo, or already having a primary of their own. If you are choosing people who, deep down, aren't completely fulfilled being a secondary, then yes... it is very likely that they will meet someone who takes up more of their attention. That doesn't always have to mean that they disappear completely - but to me, it's fair that they would shift their focus for at least the first few months of meeting someone who fulfills more of whatever need they might have.
Moving onto the 'already poly' discussion. That's a tricky one. I've met people who are already poly, who have turned out to be a nightmare because their set-in-stone ideas about how poly has worked for them so far have clashed with mine. For me, it hasn't been so much about poly/mono backgrounds - but more about their present situation. When I date single girls, they tend to cling to me, or use me as a surrogate girlfriend until they find one. I think really, it's not so much about poly or monogamy... but more about your needs and their needs meshing.
Finally, onto guidelines. I do not have ground rules - I only have personal expectations. I put them across and ask them to put theirs across. Then we discuss and we don't finish the conversation until at least a happy compromise has been made on each point.
My personal expectations for secondaries:
1. As long as she is respectful towards you, I expect you to treat my primary GF with respect
2. I do not expect, or want, you to be monogamous to me
3. Before we sleep together, I want to see the results of your most recent sexual health checkup (non-negotiable)
4. I am not looking to fall in love, I am not looking for a 'second girlfriend', (that one is personal to me and my current desires)
5. I expect you to tell me if you sleep with someone else, for health and safety reasons
6. I expect you to be honest with me at all times - if your feelings change one way or another, if you're in too deep, if you're unhappy, if I ask you a specific question, etc.
7. I expect you to be courteous to the guidelines of my primary relationship and to tell me openly if something isn't working for you
That is it. I only place guidelines that I would be happy to adhere to myself, if I were someone's secondary.
One thing I have found very helpful is to ask secondaries their expectations and guidelines, before I state mine. It can give a good insight into what they really want, vs what they are agreeing with simply because they want to be involved with you.
Sometimes a secondary isn't happy with a guideline. For example, one of my primary relationship guidelines is that we do not do overnight stays with secondaries. This is because we want BDSM play partners and FWBs - we feel overnights create an intimacy that we do not currently have room for. My first secondary challenged this and I insisted it was not just for my GF's comfort, it was my personal wish. I ignored the warning sign that she needed something more from me and carried on seeing her for 6 months, until it became obvious that she was becoming extremely attached to me. So, there's an example of me, in a primary relationship, telling my secondary how it goes, without listening properly.
Overall, it genuinely could be the luck of the draw. Relationships do end, especially non-primary ones. To me, that's the point of dating. I look at each new relationship as "ok, let's try this one out for 3-6 months", rather than "yes! this is the one! She's going to be long-term!" So... I'd continue with a positive attitude, take things slowly, be very clear about your expectations *and* get them to be clear about theirs. Compromise. And be happy to learn from each new dating scenario.