I'm going to move this thread to the New To Poly board, as it's likely to spawn a discussion and is better suited there.
I'd say that the first thing to consider is that this is not the end of the relationship, necessarily, and that the pain will pass. Your situation will get better as you work through it. Accepting the pain as only temporary can help make it much easier to deal with and less of a strain on your body.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.