I LOVE THAT..."his teeth are rotting from all the lies he spills out" My ex used to have beautiful teeth, now I understand why their so ugly now and he doesn't dare show a teethy grin.

Gum job

f.t.w!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I'm no good at jokes, but I CAN tell you the answer to your post title.
The reason you are heartbroken is NOT because you want, or even respect him. You are heartbroken over the loss of your hopes, and of the loss of what you had THOUGHT you were building with him. That's quite a wonderful thing to have thought you had, and you are well justified in your anger, and your heartache over losing it.
I don't think you will, but some people in your situation react to their pain by suddenly doubting themselves, and even though they spent weeks decrying him to their friends, they run back into the arms of the bastard. That's for the same reason. It's not about the guy, it's about what you wanted from life, and have had to give up on, at least for now.
You'll be fine.