There is no "nice" about being someone's planb their second choice, their " fallback" position. Have you asked yourself why that is good enough for you?
I take from you being here that you have read the devastation created by people cheating.you know it for yourself as a person cheated on. He is cheating with his New partner with you? Be it emotional or physical. Do you realise you are now potentially supporting him to cheat.
Look so much of my behaviour in relationships has been so far from the person I thought I was, the values I hold. Check out your values. Is this person who is in relationship with this man really who you are? I would guess it's not, but only you can answer that question.
How you expect your daughters to value themselves in their relationships with men. What you be advising your daughters to do the day the come to you and say " mum he cheats on me all the time, He comes and goes as he pleases, I feel taken for granted and abused". What do you say?
PP said it right you have all the control here over YOU and the sooner you put your drug of choice down (meaning your ex) the better you will start to feel.
I'm going to keep posting 4mykids. I'm gonna keep being your cheerleader.