'Happy Families' it is not. Do you think this baby was planned ? Not by him it wasn't. Convenient contraception failure ? Maybe ? I don't know to what extent she really wanted him as a person, a partner etc.
It was shameful and utterly selfish on his part how he dumped you, his fiancé of six years. He will do the same to her soon as well.
He's got nothing but pressure: a new (unplanned) baby (feeding routines, sleeping routines, nappy changes, how to dress the baby, feed him, bathe him, wipe his face, find clean pyjamas, favourite teddy, favourite story etc etc etc) a new partner to get used to i.e. shared values, common interests, a whole host of her friends and family to get to know whether he wants to or not alongside being a new Dad, recently moved into a new house. He is under financial pressure like he's never known before i.e. baby + Christmas etc etc.
Does any of this sound like fun ? Not to me is doesn't.
He might ruefully look back on the calm, 'just the two of us' lifestyle he had with you and the cosy familiarity. It either won't be long before he's whining to you for sympathy for his wrongdoing to you or within a couple of years, he will have another baby. But not with her. Or you.
Over the years, you will just metaphorically stand there grinning as he crashes in and out of various relationships.
The phrase, 'where have you left the one you left me for' comes to mind. It's a line from a song but valid never the less.
Wallow in your grief if you want to. It's part of the grieving process. Hopefully you will see your own, 'new dawn' one day and make a plan to move on.
Sorry you've just got sheaves and sheaves of unanswered and some unanswerable questions.