This happened to me many years ago. There was no OW involved, so don't automatically assume this is the case.
My ex simply fell out of love, so he could either carry on stringing me along or end it so we could both get on with our lives and start afresh.
Funnily enough it was me who met another partner first. Ex dp was on his own for a long time.
It was totally the right decision, even though at the time I thought my world had ended. I was devastated, but got my act together, started seeing that what we had wasn't perfect, and when I thought about it there were lots of things I wouldn't put up with now. So he did me a favour.
Few weeks after he left he came back crying, said he had made a mistake and I should give him another chance. He begged.
Obviously the grass wasn't greener on the other side.
A couple of weeks earlier and I would have loved to have heard these words, but after the time to think about our relationship I knew he was right in the first place to end the relationship.
My life has changed so much for the better. 