Could it be possible that since you both cheated that you are just expecting him to do it again? It could be that because of past infidelities you both have created a hypersensitive environment where every little thing seems to be a prelude to an affair. Nowhere in your post did you say anything that indicated he is cheating again. Just that he spoke to (I'm assuming) an old flame.

I cheated on my wife and we worked through it. One of the things that we had to work through was the guilt and suspicion. Every time I spoke to a woman without her there it felt like I was doing something wrong even though I wasn't. On the flip side every time I did something slightly out of the ordinary she began to think that I was leaving or seeing somebody else. This continued cycle did nothing but create chaos and we had to confront it. One of the things the Councillor told us was that perception is reality. If she thought I was going out and sleeping with someone else then it didn't matter if I was or not because the thought was already there. She said to concentrate and the solid things that you could see and not worry about what you thought might be happening because it would just continue to drive a wedge between us.

You can't expect him to never talk to another woman again just as you will talk to other men. Just because you interact doesn't mean anything is happening.

All that being said, if you don't love him anymore and the kids are gone then it may be time to leave and begin to build a new life. The only person who can decide that is you but the best advice I can give you is this; Do not leave over something you think might have happened or might happen in the future. Leave because you are truly not happy.