"My wife is marvellous and I have zero intentions of every leaving the cosy set up I have at home but I really fancy putting my cock into somebody else" - said no cheating scumbag, ever.
But that is quite likely to be the truth. OP you are totally invested in believing the myth that he is in a shell of a marriage to women who is undeserving of his greatness and it is this feeling of superiority, of snatching the prize away from this women, that has fuelled this relationship for you. You said it yourself, that you were flattered by the fact that he used 'her time' on you (not quoting but that was the gist). It's not just about him picking you, it's about him picking you over her. You need to feel 'better than' and it's a bitter, bitter pill to swallow that after 10 years of proving that you are the iwife 7.0 version, he still wants his old model (and you do want him in that way because there is no way you would have let him play daddy to your kids if he was simply a fwb).
You're not going to end it with him. Your ego has taken a bashing because you just can't find the moving his healthy mother in thing to be anything other than an excuse to stay with her, so you need a fix. You'll tell him it's over, he'll waily, waily over how he can't live without you but he has to stay... Because it's his duty you understand, not because he wants to and you'll fall into each others arms (passionately, no doubt), and on it will go for as long as his wife has no idea about you.
Stay, go, do what you like OP but please leave this women alone, she has done nothing to warrent your vittriol against her other than to have met this man before you. You don't know her and you have no idea about the reality of their marriage, only what he has told you and he is a liar.
Leave her alone and sort yourself out.