Hi OP
I have just read this thread and being the wife of someone who had an affair I can tell you I would definitely want to know BUT not from the other women because in your case it is obviously sour grapes on your part that this bloke never chose you.
Most men never choose the mistress and if they do then there is a vacancy for the next mistress as men like this rarely change, so unfortunately I am sorry but he probably will fill your place with someone new.
I understand you fell fir this man in a big way and truly believed he wanted to be with you and I know you believed every word, I am guessing you are young and thought he was the one and he wasn't.
Your pride is hurt that he wasn't bothered you ended the relationship we understand that too but your telling his girlfriend is your way of trying to hurt him, not inform the girlfriend!
If the mistress had rung me to tell me she had a relationship with my husband my natural instinct would be to stick by my husband , why? Because I would think you we're like Glen Close in that Movie with Michael Douglas , a bunny boiler!
Please don't get me wrong I am not dishing you for following your heart at what you thought was the real thing, you were fooled but now it is time to walk away with dignity and put All this down to experience and move on and find someone Single who will love and respect you as you deserve.
The best thing you can do is forget as all you are doing is festering and getting more obsessed with the situation.
There is a whole wide world of lovely Honest men out there, and one of them is waiting for you!!