Throw away or burn Ester Perel.
I think Esther Perel captures the “why” better than anyone else I’ve seen, and actually her stuff aligns directly with mainstream SI-thought. I think the problem people have with her here lies in thinking knowing the “why” somehow justifies the “what”. It doesn’t. This thread has a bunch of thoughts, including my post on it (minority opinion).
https://survivinginfidelity.com/forums.asp?tid=611574&AP=21#mid7984894
Of course a person can have feelings for more than one person. For thousands of years young people have had multiple suitors or multiple muses, and having to choose between them and dedicating to a committed relationship. There’s no switch that gets thrown making them disappear, either. Those memories or thoughts of desire are, however, something that can be fed and grown through conscious choice.
The simultaneous hunt for security and excitement, the known and the mysterious...old as humanity. There’s a reason romance novels are the number one genre of books sold on the planet, read by females predominately. That’s pretty much the plot basis for all of them.
What your partner did was simultaneously excruciatingly painful and yet totally typical. I’ve seen so many marriages involving first-and-only partners get hit this way. For every one that blows up, what percentage involves them at least contemplating it?
One of the girls I dated in college had been with her boyfriend through high school. We dated while she still dated him, and they eventually got married and have been so for 30-some years. I hope I contributed to the solidity of their marriage by making the mysterious mundane. 
The analogy to the military and being separated lots...it is a fact that divorce rates in the military far exceed civilian populations. Either military personnel are really, really bad at picking spouses, or separation places stresses and temptations that end up crossing different limits for different people. It takes a special spouse to endure and thrive in those types of relationships, simultaneously independent yet committed.
The thing is, you can understand all of this completely, and it essentially does nothing to heal the pain. Might even make it worse, in that you wonder why it hurts if you understand it. For that, you have to look inside.
This I know for sure, though. Your relationship is changed forever. There is no going back, only going forward.
The other thing I know for sure is that the key to moving forward for you is to take control of the outcome by recognizing what you can control (your actions and can’t control (her thoughts, feelings, actions) and work from there.
Sending strength!
