Anonymous wrote:OP here: it’s not like she was a one-night stand. He had a long relationship with this woman, and it took a LONG time for us to repair our relationship when I found out. I feel like him accepting her request is allowing her back into our lives. Its not like he decided to leave her because he felt bad...they got into an argument and she told me about their relationship. Otherwise, I would have never known....
This post says it all, OP. He did not feel guilty and come clean out of love for you; he would have continued the affair if she had not outed him.
Read that again: He would have continued the affair if he hadn't been outed. He had no intention of dropping her because cheating was wrong, cruel and immoral toward YOU; he only dropped her because he got outed to you. He got off on running two women at once.
The fact you took him back is water under the bridge but it's past time to kick him out. Have NO regrets. You already invested too much time in him. It is not at all an overreaction to end things because he had contact with her.
Please block him entirely. Do not respond to texts, calls, anything. If you have mutual friends who would tell you you are overreacting--dump them as friends because they do not have your best interests at heart. Do not cave in if he tries to win you back with "romantic" gestures or tears or "remember our good times" crap. If you were living together, get out immediately even if it costs you financially. I worry they you will start listening to friends or family who blather about how great he is, how hurt he is, etc. Don't. Get out and find new activities and double down on your truly supportive friends.