Hi Iggy
I am in a similar(ish) sort of situation although further down the line from you and now going through divorce.
I hope this doesn't sound harsh, but this is so common of the shite that these men spout. They act as though some sort of tragedy has befallen them, they are sorry for the pain they've caused but 'can't help themselves' etc.
There is however a very simple truth behind their actions. They didn't love you enough any more and so decided (yes, actually made an active decision) to start screwing another woman. And now the relationship with the OW means more to them than your relationship does.
It is a horribly painful reality to engage with, and I still struggle myself, but in my experience, it is better to realise this sooner rather than later, or your mental and emotional health will suffer horribly.
My STBX actually said that he 'wanted to have his cake and eat it', but guess what, that would destroy you as a person. You need to realise that he is no longer who you thought. He does NOT want you anymore, but wants to assuage his guilt. But really, it is simply not that difficult, if you love someone, and love being in a family you stay with them and don't start seeing an OW.
I hope I've not been too harsh, but I wish I'd realised this truth sooner, and I wish I could really 'believe' it too, rather than just 'knowing' it iyswim, but I am getting there.
Good luck, this is such a shit time so sending huge hug xxx