I hate my partners mother?

Reading between the lines it would appear that is why your partners ex.left him.He is a mummies little boy and always will be.He has got to tell his mum that if she is going to carry on treating you the way she is doing he will not be visiting her again until she changes her attitude towards you.I doubt very much whether she will change though.I personally was in a similar situation.I split up with my wife and filed for divorce.I met a lovely girl [to whom I have now been married 36yrs] My mother blamed my now wife for the divorce[which she had nothing to do with] and made my now wife so uncomfortable I told her we would keep away unless she changed her attitude.We never went back to see her and I learnt that she had died.Guilty conscience yes,but I had to stand by my wife.