I just told my WH the other night that the TT period (which is still ongoing, he won't tell me shit) is what really killed any chances.
I think part of is is that they think that if we know the truth, we'll definitely leave. They don't understand that the wondering, etc is way worse, that our imaginations are way worse.
I think the reason it hurts so much is that it adds to the sense that the trust we had can never come back. That feeling of being a single unit together, of sharing everything, is shattered. And every single time they cover up, or minimize, or deflect, it's like they are grinding that shattered trust under their heels a little more.
The TT absolutely is worse than the affair. I can understand being caught up in a whirlwind of emotion, self absorption, etc. But once reality/Dday hits, to CONTINUE??? I can't ever get past that.