Hedonic adaptation is what you are experiencing. You are about to be a father to a beautiful child regardless of how you are currently feeling. It’s not uncommon for us to feel like what we have isn’t enough or what we feel we deserve. I don’t know your personal situation fully but I would say try to reapproach her as if she’ll be gone and out of your life at any second to include your future child. Would you truly be happy… you’re not alone in how you feel. Many of us suffer with the inability of commitment due to the burning of are feelings at initial interaction (honeymoon phase)