Oh I am so sorry sweetheart. It's so heartbreaking when you love someone and they get with someone else quickly and move on, doubly hurtful when you are carrying their baby. It is exceptionally hard to just 'move on' ( as people used to tell me, and my ex used to say it to me!) when this happens. The only true thing that will heal this pain is time. We cannot make someone do something, no matter how bad their actions are, when they don't want to. Firstly, accept that you feel really bad and just indulge, eg cry a bit, eat ice cream, listen to music, slob out for a week or two and then pick yourself up and concentrate on this baby and the delivery.
Giving birth is a special, special time in a woman's life and it's crap when the father is not there to support you. However, he has not earned the right to be there and he certainly has not earned the right to keep upsetting you now he has gone. Try really hard to enjoy this baby as he/ she will take away this hurt and it only gets better as time goes by.
I was like you, so so hurt and devastated with my ex. He is now playing happy families with another woman and letting her kids call him 'Dad' despite telling me he couldn't cope with kids and he lied about why he had to leave me. He was just a coward and I am glad he is not in our life as he is unreliable and extremely selfish. I still have bad days when I think of him but that's more to do with not having another partner since rather than missing 'him'. You may feel like this, just brush it off as just missing male company.
He is obviously a cheater and I am sure his ex will experience this infidelity herself as he is sneaky isn't he and they broke up for a reason once before.
Be strong, seek any help you can from friends, family etc. you WILL cope and get through all this, it just takes lots of time, don't be too hard on yourself.
Hugs xx