Close friends hid the fact he has been cheating on me (advice needed)

Background: I’m 26F engaged to 30M, together for 4 years now. Originally an open relationship, but he closed it the first week of us dating. We conceived a baby by accident in December of last year, but I unfortunately had to terminate the pregnancy for health reasons. No kids. We rent an apartment together. We share a friend group. Both of our families live close-by.

Yesterday morning, a close mutual friend of ours (who I’ll call Jane for this post) invited me to her house to “discuss something important.” Jane then begins to give me a timeline of the last six months where my partner has been having a relationship with his co-worker, 18F. In January, he visited her at her apartment when she accidentally locks herself out. He decided to rent a hotel room for the both of them and stayed the night with her. He informed me of this and swears nothing happened that night. In April, he went to our mutual friend’s bonfire and was witnessed by multiple people getting drunk and kissing and groping 18F. They allegedly then went into a treehouse by themselves on the property and were quiet the rest of the night. My partner also told me of this event, but left out the detail where he kissed and groped someone 12 years younger than him. He has had various other outings with 18F, but no other romantic/sexual interactions were witnessed. I was aware of most of this timeline except for the two of them kissing. There were red flags, but I chose to ignore them like an idiot.

After Jane is done, she then tells me she knows about the baby I terminated in December. Jane was told this my another mutual friend (who I’ll call Ashley) who was told this by my partner. Ashley felt this was relevant to why my partner was cheating on me, hence why she told Jane. Jane, for whatever reason, told her partner (Ian), again another mutual friend. She’s states she told him because she “tells him everything”. I was keeping it together up until this point, but I left immediately after she said that. I am so ashamed of myself for what I did. I wanted to keep that secret to the grave, and now our entire friend group knows. Choosing to terminate our baby was extremely difficult for us both, yet I asked him to please keep it between us for now, and he agreed. I know, it was a shitty thing to ask of him. I was depriving him of the support of our family and friends. But I just couldn’t cope with myself and my actions yet.

Jane, Ashley, and Ian have been pestering my partner about coming clean since the kiss. He had told them that he told me everything early this June, but Jane did not believe him, hence the one on one with me. They asked him to stop talking with 18F as well. He still does to my knowledge.

The cheating pales in comparison to how my friends handled this situation. They did not say anything to me for nearly three months until yesterday. Telling one another about the abortion is just unforgivable to me. It really feels like they reduced one of the hardest events of my life to a good piece of gossip. I don’t even know how to begin approaching this issue with my partner, my family, and my “friends”. The only concrete plan I have so far is to have a discussion with my partner later today, and start therapy. Should I cut him out of my life? Should I cut my friends out?