Op, this is a very tricky problem to respond to because you are obviously in an abusive relationship, and yet you don't know it.
This man has manipulated and gaslighted you to such an extent that even being abandoned ten times hasn't woken you up out of his spell.
You are focussing on "her" as the problem, to the extent that this narrow focus has blinded you to what is really going on.
And that is, he's playing the both of you, because he enjoys it. He probably tells her the same lies, that you manipulate and blackmail him into coming back.
Six pages of responses so far, and every person on this thread can see it, because they can see the bigger picture, and they have seen this pattern of behaviour in abusive men before.
I know it sounds like a cliche, and it sounds like a hard thing to do, but the only person who can end the abuse is you.
He will keep you both on his puppet master strings for as long as he wants. My prediction is that he will bring a third woman into the equation at some point, and you will both blame HER for "stealing" him away.
Please stop being his puppet.
Cut those strings today and get your dignity back.
Pack his things into bin bags and change the locks. Don't ever let him into your home again. Then you can begin to heal and to make sense of what REALLY happened over these past two years. Currently your understanding of the situation is entirely dictated by him.
Is there anyone who can come and stay with you for a few weeks? Mum, sister, best mate? Just for moral support and to talk it through.
What do you family and friends make of the way he behaves?