Not a BS here, or a WS. I'm here because during a years long major depression, I committed a kind of emotional abandonment of my wife. When I sought help I found that the strategies some people used to recover from an affair help me understand how I might help heal my marriage from the break in trust that came with emotional abandonment. But I want to be clear I haven't really walked in your shoes, so take my thoughts for what they are worth.
BW just asked me about feeling guilt for my AP's partner, a man who himself had a 4 yr old daughter to a previous partnership. ....
Whats wrong with me? Why arent I connecting with that side of it? ...
I should note that looking back over every significant female relationship I have ever had, I have been an OM more often than I havent. That's scary.
I singled these parts of your post out because I think they offer a key to reframing the question.
I said to someone the other day that thinking about what I have been learning, it seems that "The say in war the first casualty is truth. In an affair, the first casualty is empathy, and it dies long before the affair begins."
I think what you are struggling with, and what your wife is groping to help you focus on, is not lack of guilt, so much as an impaired capacity for empathy.
For people who do not engage in affairs, even when things in their relationship are not good, one reason is a healthy sense of empathy. They reflexively think how their spouse would feel should the prospective disloyalty be discovered, and they flinch from the anticipated pain they know their loved one would feel. That anticipation of future pain given to the loved one is part of the "animating force" that helps make boundaries stick and helps you make good choices in the beginning, and sty off the path of rationalization that leads into an affair.
The fact that you have often positioned yourself as a third party to your partners' relationships suggests that this healthy mechanism is impaired for you.
The good news is this awareness is something you can work on. I do believe people come with different degrees of natural capacity for empathy. But even if your capacity is lower out of the box, as it were, you can choose, as a matter of active work and self-improvement, to cultivate it. Monitor yourself for red flag thoughts, like "what they don't know won't hurt them," or, "his/her relationships are her responsibility, not mine." When faced with a choice, action, or temptation, ask yourself, will this choice build happiness for everyone affected by it, or will it help me at the direct expense of others? And this is not limited to marriage and romantic relationships--it applies to business, to civic life, to your choices as a man in the world in general.
So I would set aside guilt per se. I would reflect on the fact that your pattern of relationships, and your difficulty even now in getting an emotional response from yourself when you think of the risk of pain your choices exposed the AP's innocent stepchild to, reveal that your boundaries around yourself are very tight, and your capability to use imagination to project yourself into the feelings of others, and feel their feelings as being as real and valid and morally worthy of your consideration as your own, is underdeveloped.
So come up with your own strategies, with your IC or on your own. But develop a set of exercises or a list of questions you can use to check your decisions and behavior, and test them to see whether you are taking them having actually considered their impact on others in the world.