So sorry for what your going through.
I personally don’t believe in karma.
I can relate to your situation though and maybe able to share some advice. At 25 I got in to a relationship. One night 2 years after we got together a friend bravely told me he had cheated on me with a girl I considered a friend of sorts. This had been 9 months previous and the baby she had given birth to was his.
It’s all so clear now but at the time it was like a nightmare. I left initially, but went back. So many reasons for this, I couldn’t bare it if they had got together and I had given so much why should I lose it.
I to, often told friends I was just staying for now etc, that’s because people judge if you stay.
For 6 months I was obsessed with her social media, pictures of the baby and wondering why her. He tried his best to prove he would be faithful but my insecurity was so intense.
I often thought I must have been a bad person to deserve such pain. Eventually I went to counselling and started to understand forgiveness (not easy when your so angry). It took time but I realised the only one holding on was me.
I did manage to stop checking her social media, now we have mutual friends and I smile when I see pictures of her husband and children. I understand we are all searching for love and belonging and that allows me to let go of the resentment.
As for me I did leave the relationship after 3 more years. He’s married with children.
When I met my husband we were both in relationships, there was no affair only a realisation that the relationships we were both in were not healthy. We ended them and then began dating.
My husbands ex believes we had an affair and now hates me like I hated the OW. Despite always conducting myself with dignity and self respect I am viewed by another person with contempt.
Choose forgiveness, not for them but for you so that you can move forward with a light heart and eyes on the future.
All my best XX