My ex left for another country (well, Ireland!) when I was pregnant. He convinced himself it wasn't happening, then we had crocodile tears and promises he would be there for me too.
To get through it I think you need to psychologically discount him. If he goes or stays around won't affect that you are the main (if not only) carer, so be practical and put things in place about how you will cope on your own with a baby. Can you manage financially?
Emotionally, my son is now 2.8, he sees his dad every 6 weeks ish, he never asks about him or lack of him - it's just his norm. I suspect it might change at school when he sees other family set ups, but your child will prob be the same. It's just their normal life, so don't worry about trying to explain things until much further down the line.
I try very hard to focus on the positives - I want my son to have great self esteem, so to try and counter any feelings later down the line of why doesn't my daddy bother with me, I tell him a lot how special he is, how much I wanted him, how lucky I and others are to have him in our life. Sorry if that sounds wanky, but if it helps I thought it was worth saying it.
Good luck, and congratulations on your pregnancy 