What if you started with something really small that hurts/bothers you? The example you've given is quite large (cheating wife) and really not a good starting place.
Don't try and run a marathon if jogging around the block tires you out.
So, the next time something small and annoying comes up, call the person on it. Don't think about it, wait on it, just do it.
Also, do you have the same problem with men? Just wondering... I know I personally find women much more intimidating. So if you're like me, maybe try calling men on things first, as an easier lead into dealing with women in similar situations.