When you mentioned feeling like you were her dad we could have seen that one coming a mile away. Not having a dad treat her like his little princess denied her validation and love in her growth years. When you entered the picture you might have expected to be a friend, a lover, and even a father to your child.
Without prior knowledge you couldn't have seen that she grew up with a void she expected you to fill. She knew whe wanted you as a friend, lover and maybe father of her child ... but she probably never realized she also needed you to fill that hole from her youth. She needed you to be able to make her that princess. You may have succeeded at times, for both of you. But she expected forever. She expected it to go away, yet it keeps coming back. The OM won't be able to make it disappear long either. No one but her can confront it. All anyone else can do is distract her.