For me I don't think the living child is going to replace the ones I have lost, my first should be a 5 year old, and for each of my losses I still think about the milestones we miss together, and mourn them. It sounds like what you went through was incredibly traumatic and it will be hard to heal from that, but it was absolutely not your fault. Yes you should have left but there was no way you could have seen the future in the moment, and as someone who has been in an abusive relationship it is so hard to leave, even without being pregnant. I'm sorry for your loss, but I am glad you have your rainbow baby and I know he will be a joy in your life. No amount of love you have for him will take away the love you have for your angel baby
You should.know that your feeling are valid and you have every right to be sad and cry and miss her. She was a life you had inside you. Your body still went through postpartum. You are fine to mourn the lost. I think this is natural. Also I will say I would consider talking to someone about healing from this. There are people called bereavement doulas that do this. I wish you luck mama and congrats on your rainbow baby