Well, you went on to explain that "I mean when I am reasonably sure she has seen him, or is with him, etc, I get all those strong feelings about ending things, lashing out, etc.". That's indicative of not being detached, so your question "Is it strength when I can only maintain my cool/detachment so long as the affair is not staring me in the face" seemed answered by yourself. You don't see it?
It's easy to be "strong" when we're temporarily OK and not presented with grief (when it's not 'staring you in the face'). The true test comes, like most things, when we're faced with the task and the old behaviors well up in your core and pull at you... what do you do then? That's the true measure.
It's like old habits (which is what behaviors are). It's easy to not smoke while you're sleeping or taking a shower... but what do you do when you've just finished breakfast, or are drinking, or a friend of yours lights up? That's the measure of how strong you can be. The notable thing is that, even if right now you're not so strong, you can make yourself stronger in the face of adversity.
