I don't think experts know why or for that matter that anyone knows except maybe God. I ask myself that all the time. I think I've just become a by-product of his abuse. It's left me with such feelings as you yourself must be feeling. When you say there are good times, there certainly are.
Abusers are probably some of the nicest, most charming, fun people to be around.
But when their other side emerges, it's like being in hell, literally. He's driven me to become what he is. He's created in me, what his father created in him. Fortunately for me, I don't have children. I shudder to think what that might have been like.