So sorry for you OP. I have just joined MN specifically to post as your plight really speaks to me.
I am on the other side of a similar situation, (10yr relationship , just when I got my devastating fertility news) and I just wanted to offer a few words of support...
I know that on top of the betrayal, you are now contemplating huge grief for the future you had planned and the family you hoped for so desperately. And to make matters worse, he forces you to be the one to walk away, to make it your decision. But like the general consensus, I agree that he is the one who has well and truly shat all over that.
What comes next is the hard part, but I want to give you some moral support in that regard... I know the concept giving up your life plans for a flatshare is seriously unappealing. But if that's what you decide, I'm telling you it can be done, and honestly, my two years in my post-split flatshare were the most fulfilling, fascinating and FUN years of my adult life. My only regret is how long it took me to leave.
I know when your fertility is at stake striking out alone does carry a terrifying risk,but like others have said, if the man you're depending on is capable of that behaviour, after everything you've been through already, he'll only ever disappoint you one way or another anyway.
And you're right, it's easy/tempting to remember the good stuff, but it's funny... when you move on you will begin to see all the little things that were wrong wrong wrong.
Anyway, I know my case is just anecdotal evidence, but for me, being free of a sulky, lying egotist gave my life a new zing. I had a ball, and the gods smiled on me and introduced me to the most wonderful man, and two amazing years later we are married and expecting. It CAN happen.
He may have let you down, but you don't have to let yourself down. Look to the future. There will be dark times, but try not to give in to hopelessness. Imagine how much brighter your life will be. I sincerely wish you all the happiness you deserve, and all the courage and strength to get there.