Marital infedility can be one of the most difficult things to deal with in your adult relationships. It's VERY difficult to be able to trust someone you cheats on you (whatever their reasons are/were). And in your case, Doug, it's happened MORE than once. 
And the problem with taking someone back after they've cheated on you is... they come to realize that you'll take them back, no matter what.
At some point, you're going to have to ask yourself the difficult question: is the relationship worth the aggravation? Is it worth the feelings of betrayal?
As painful as it might be to do... you might consider cutting your losses sooner, rather than later. If your partner is not committed to the relationship, there's nothing sadder than realizing that you're the only one in the relationship that gives a shit.
That said, I know it's hard to "give up" on something that obviously means a lot to YOU (even if it doesn't mean much of anything to the other person). Still, while I don't really know you... I do know that no one deserves to be treated like that.
Good luck to you, Doug, in whatever decision you make.
