Sex is such a double edged sword. On on hand, it is a way to feel closer to a WH, to make a connection without words. On the hand, it brings out feeling of sadness, anger and resentment as it is a reminder that he shared these intimacies with someone else. For me, on occasion, I found that I would either pick a fight with fWH the next morning or find refuge in anger as I was feeling vulnerable. As time passed, and I saw how fWH was changing for the better I was able to deal with the emotions following sexual relations.
Be gentle with yourself. It is hard to share yourself with someone who has hurt you badly. What you are feeling is normal, especially when you have waited to resume a physical relationship. As time passes and if you fWH continues to do what you need him to do to help you heal and fix his issues that led him to his A, the resentment may subside. It may help if you share your feelings with him.