Woman having an affair with a married woman...

So... we fell in love suddenly somewhere in 2016. She is married, has a 20 year old son and an emotionally abusive husband. I understood a lot of things - that she cant leave cause there is nowhere for her to go except for her mothers place. That she doesnt want to come out because of her son. That she has to sleep with her husband every 2 weeks cause otherwise he makes her life a living hell. During these years we spent time together as "friends" - he knew she was visiting her best friend weekly - even that caused fights cause "why doesnt she want to stay at home all the time"? Also, he is accustomed to the idea that the woman must do everything around the house and he must just sit there, drink beer and watch TV which pisses me off so much... she lets him treat her like crap and complies with everything. He doesnt even have a job, works day to day wherever he can and he still humiliates her for having a 9 to 5 job that doesnt pay well. Anyway, during these years, I talked to her about this and she slowly made progress. She talked back to him, she didnt let him get away with it so easily. She stopped sleeping with him as often until she didnt sleep with him at all - about 1 year now. Last week he gave her an ultimatum - she either sleeps with him or he kicks her out - the house they live in belongs to his family. She talked to her mother about it and her mother agrees with him (she's old and has a very strict mindset - the same as his). Now she considered leaving him and go to her moms but she knows that she wouldnt be ok there. So she agreed with his ultimatum. Im depressed for a few years now because of this, I dont enjoy life and friends anymore.
So now I'm left devastated and I told her that I can't go on like this again and that I should get out of the way. She doesnt want to break up but I cant take another day of wondering if she is sleeping with him or not, regardless that she does it as an act. There is no future for us since she doesnt want to leave him. How can I be ok with moving on? What can I do to make this pain go easier? Thank you