This may not be accurate in your specific case, but
I never thought about how this would hurt her.
is probably not 100% true. Most of us knew our A would hurt our BS. Maybe we didn't realize the true measure of harm, but we intellectually knew it would be bad. But there is something in us at the time that allowed us to push that to the side.
I have said in the past that if I realized how much it was going to hurt BW, I wouldn't have gone through with it. I am pretty sure that is a lie. I would have had to rationalize and compartmentalize even harder, but I am sure that I would have sunk to those depths to do it.
The same thing with "I didn't know what to do." We knew what NOT to do, but something inside disregarded that voice.
It doesn't do any good to forget these spots. You have to open them up. Those are the areas where you need to look. It doesn't happen all at once. Lots of people here pointed me in this direction. I owe them a huge debt of gratitude. Not that I have it figured out, but these kinds of observations helped me really look at myself. I hope it helps you.