Read your story, just wanted to say I am so very sorry you had to go through this and at such a delicate time as well. I can't begin to imagine how you must have felt. I hope you're ok and your baby girl is doing well too.
I don't really understand why someone would just walk away like that, but sadly some do.
My sister went through similar a couple of years ago, only her partner had been cheating and had had another child whilst she was pregnant with their daughter. She found out days before her due date and was devastated. He said he wanted her and he was sorry, she gave him another chance. When their daughter was three weeks old he flipped and left her for the other girl. She's not seen him since and he's not bothered with his daughter since and she is now two and a half years old. To be honest, she is so much better off without him, he had played games for their entire five years together and she is so young (now 23) with so much future ahead of her, she doesn't need someone like that in her life, neither does her daughter and neither do you and yours. She is doing much better without him, much happier and much more confident. I hope this is the case for yourself too because you two need good stable people in your lives, not someone who'll just up and leave like that.
Anyway, much love and hugs to you, I hope you're both ok x