How does the circular logic of 'the A is keeping me from seeing my marriage realistically and having hope in it but I am not going to give up the A because I don't see hope in my marriage' ever get broken?
Ah yes....that's a good one isn't it. The problem is, a "real" relationship can't really compete with an A now can it? There's no problems in an A. There's excitement, there's fun, there's all the happy fun stuff with none of the typical day to day "crap."
Even in my case, it even went one step further...not really the "happy fun time" argument at the end, but rather, more of a "just in case" type deal. Ultimately, that drove me away, and then she picked right back up with OM (which basically proved my point).
So what can we do? I think ultimately, we give them what we want. We let them move on with their life. We let the A run it's course. There is a lot of soul searching that needs to be done by the OP, and I don't think that can really take place when they are involved with someone. When the happy fun time turns into a daily grind, at that point, maybe they start to see things differently. Then again...maybe not. There's just no guarantees.