Actually, if I were you, I would react the way I posted. Then, I would think better of it and do the following...
Go back to being the way you were. Forgive your H, forgive yourself, and forgive the OW. You don't have to like her or forget her, but maybe you can find something to pity her for. That always works for me. My H did not have a PA, but I did find plenty to pity the EA OW for. It would be harder for me with a PA, but I think I could do it.
Then, just be yourself. Be around, but not to spite her, rather to just get back to being yourself. If it spites her, well, you won't be paying any attention to her anyway, so who cares? Did she take your feelings into account before taking up with your H?
Being happy, confident, and feeling free, do what you need to do and want to do and don't pay any attention to what does or does not affect her. It will feel mechanical at first, but keep being yourself and making yourself happy until it is second nature.