You've heard absolutely 0 from this guy, his actual thoughts, his feelings. So far, all I've heard him say is that he's confused. Which for a guy, admitting that you even have feelings probably was a confusing step in its self.

Want to know why she would stay with him? Because she loves him. Because she cares for him. Because she can see he's in between a rock and a hard place, and the correct response for a friend and a lover would be to guide them out of that spot, not just say "fuck it" and push yourself out while wedging him in further (because believe it or not, he's probably well already thought about how this is going to go).

Want to know why I feel like the OP is getting the worst responses? Because he told her. He told her about everything within a 48 hour window unless the times are off. In fact, he told her in the exact same amount of time it took for the OP to tell her partner that she felt insecure. He told her when the ex proposed lunch. He told her about their past.

He's been rather open about all of this ,and now he's in a really confusing state.

I dont see any advice saying "I see you're both hurting, so treat him as you would a friend - guide him.". I see "You're now second best, fuck the year and a half of happiness that you've spent together. This one night has ended it all".

Really? That's how little faith you have in their relationship?

For the OP - I think you've done it right, but you should be sure to affirm that this isn't the end all. He's (very likely) got it in his head that he now has to choose. There isn't really even a choice. You let him know that being in such a place is confusing as anything, and it'd be only less passionate if it were between two best friends who didn't click.

And if you ARE ready to jump and run, I honestly recommend doing it. As the support between you two shouldn't be this flaky. This should have been a very quick "I love you, and I want you to remember that. You've run into a shitty situation where your old love seems to be re-igniting, and I can understand how that must feel, I've fallen in love with you too. But I wont settle for second, and I wont let you hurt yourself by building up only to cut it out. I think it would be in our best interest to let her know that old feelings are returning, and that it is hurting. That distance is something that is best for you all. The visits were nice, and that we wish her well."