This is a difficult question. I've been someone's mistress, as some already know, and it is something I am thoroughly ashamed of.
I didn't (at least consciously) want to be a mistress, but I fell in love with a man, whom I then found it almost impossible to leave. He was my first lover which didn't help.
I was very arrogant. I didn't think I carried any responsibility for the situation. In my mind it was he who was betraying someone, and he was in my opinion 'free' to love whom he chose - but I did hate the fact he was lying and wished desperately that he would tell his wife, or that his cover would be blown, as it was just so wrong.
I have changed my viewpoint now, but only after the event. I did carry responsibility, and being in a deceitful relationship for four years damaged everyone, me, him, his poor wife, and everyone else who knew any of us. I would never, ever, EVER enter into something like that again and wish to God that I had not been involved with it.
Of course I was blamed but so was he. I realise now that part of me wanted to make a point about him not belonging to anyone, that was me beng screwed up, while he was probably wrestling some other weird demon, which he refused to look at even at the time.
He married again after his wife did find out. I imagine the new wife will be cheated on as the first was. It is like a pathology.
It took me some years to reach a point where I can say I am 'over' it, but that is to do with my own dysfunction and psychological investment rather than it having been a valid relationship.
I am sorry for everyone I hurt, directly or indirectly, but I do often see a mistress taking the flack when perhaps the man having the affair holds most of the cards. I certainly never felt I had the slightest bit of power in the whole set up. He came to me when he wanted to, left me when he wanted to, and lied to everyone including me.
I find it hard to respect women who enter into affairs with married men, but much, much harder to respect a married man who is involved with another woman. I think that's my answer.