Never married the woman, although we were engaged - no kids, thank Grud - but I’ve been down a similar path, and I have at least an inkling of what you’re going through. Some good advice in this thread, which I won’t re-hash, but beware of one thing: if you do split up - and in my view if you don’t you’re a fool or a doormat {I was both} - if this new relationship of hers doesn’t work out and she comes back with the “I didn’t know what I was doing, I was an idiot, I swear it’ll never happen again.” line, DON’T BUY IT. A break is a break, no matter how much it hurts, and if she’s screwed you over once, she’ll do it again, if you let her.
In my story, exactly ten years ago, we lived together for a few years, and became engaged: she was twelve years older than I was {I was still finishing university}, and it was a long distance relationship. We wouldn’t see each other for a couple of months, then it’d be all on for the next month, and so it continued. She kept promising to move up to live permanently, but it never eventuated.
She came back after one trip and told me she’d been seeing another guy, which I guess I’d half-suspected - she didn’t know where it was going, loved me but was so confused, needed some time to work things out, etc etc. I was completely shattered, but agreed to give her some time out: I loved her - I was infatuated with her - and grasped at any straw I was given.
After a couple of months, she came back, and I got the “terrible mistake, I promise I’ll never hurt you again” speech: we talked it through, and agreed that I’d move down to the other end of the country to live with her. And so I did. Then she started disappearing into town for days on end - we were living out in the country - and after a couple of times {yes, I know I was a fool: I knew what she was up to but hated to finally face losing her for good} I confronted her, and she admitted she’d started seeing the guy again two days after she got back. Her excuse - her exact words - was “I promised too much.”
To my eternal credit, I didn’t beat the shit out of her. I calmly called the airline, arranged a flight back up the next day, and then proceeded to get stinking, howling, bawling, puking drunk on a bottle of bourbon. She drove me to the airport the next morning, there were more tears from both of us in the car, and then we arrived at the airport. She turned to me, crying, and asked “Are you really sure you want to do this?”, and my “Yes.” was the hardest word I’ve ever spoken. I got on the plane, and never saw her again. She was the love of my life, she broke my heart twice, and if I’d let her she’d have done it again.
Sorry, Inigo: I didn’t mean to hijack your thread - just to let you know you’re not the only one out there, and sometimes the right decisions are the hardest ones to make. Hang in there, mate.