Transitioning past infidelity
Hello, I'm new here and have only recently begun to connect to other poly folks (though I've been in poly relationships for many years) so please help me out if I'm misusing a term or missing the mark on something common.
I currently have one partner- we're co-parents of a teenager who is developmentally delayed and we 've been together (across a variety of different relationships) for 14 years.
We've been pretty good about coming to the table to hammer things out when our relationship grows beyond the contract and needs an update but he's been terrible at respecting our agreements. For many years, we had a primary relationship and were simply "open" (meaning that we could date others openly as secondary relationships). Then, about 4 years ago, I was having a really hard time with my family of origin, neither of us were seeing anyone else (unusual timing for us) and our son's needs escalated. I asked for temporary monogamy (asking him to please not date anyone or bring anyone new into our family situation until things got a bit more under control). He agreed but immediately started a relationship with someone at work. He kept it a secret for about 2 months, though I knew something was up, and when I found out (NOT through him telling me), I asked to meet her. Well, she was actually pretty horrible- made fun of our son!- so I said, "You know what? No. I am not okay with this person having anything to do with you or our family as a whole." He took this really hard and never really seemed to understand that he had violated his promise and that this was a betrayal.
We changed out agreement to include a process through which we agreed that the other needed to meet someone BEFORE actual dating would happen. To sort of screen out folks and protect the family. We each did this for 4 years, though he never bothered to meet the women I dated- just gave blind approval. I veto'd one woman and encouraged and welcomed others for him.
Last February, I sensed something cooking between him and a student of his. The way he talked about her, it was obvious. So I asked more and, after considering the age difference, power differential, and the fact that she had told him about some pretty serious unprocessed trauma from childhood sexual abuse from her foster dad, I told him very clearly that he was not OK'd to pursue anything with her. He agreed and seemed to understand her vulnerability and why it wouldn't be a great idea. Last Friday, I found out (again, he doesn't tell me stuff- I have to find out!) that he has been building a relationship with her, getting very emotionally intimate, and finally had sex with her. The past week has been an insane rollercoaster.
Here's where we're at right now: he and I sat down with her to break up with her (I wanted to be sure that she was supported and understood that she hadn't done anything wrong or inappropriate and he's just been very insensitive), she has accepted this and ceased contact (past a few understandable raging emails) and he and I have renegotiated our agreement to be a closed group. It's just the two of us right now and lots of healing needs to happen before we move forward but both of us very much want to find a woman in the future to share our life with. For the me, this new agreement addresses a need I've been more and more aware of- for a third member of our marriage rather than just these casual dating scenarios on the side. For him, it might mean... well, I guess I am not really clear on what it means to him. After so much deception, I'm finding it really hard to trust anything right now.
So my question that I hope y'all can help me with is this: How might we move forward? I have a sense of how we can heal the trust issues between us but the idea of him bringing someone in is uncomfortable to me because of his poor judgement. Also, how DOES a couple go about dating women seriously? I feel like we've really stepped through a door here and I'm very excited and hopeful for our future but wow do I feel dumb about this all of the sudden!