So the bastard had an affair while you were pregnant, and now wants to leave the family because of your faults. Really? YOUR faults? I'm not buying it.
If you want this prize specimen back, your best option now is to toughen up and do the exact oposite from his expectations. Tell him to move out. Tell him fine, he wants to be single, he's single, and that's his look out. Lawyer up. Wise up. Don't fgs talk about 'fighting for him'. Like he needs the ego boost? Glam up, go out with your friends and start rebuilding your confidence, independence and joie de vivre.
He'll hate it that you are happy and coping without him. He might also see that woman he fell in love with.
If he hasn't already emotionally left you I bet you any money he'll start to panic a bit about what he has thrown away. If he has, the only difference it will make is that you will have saved yourself months of miserable pining and stress over his actions, and will be well ahead in rebuilding your life.
Finally, get a job. Doesn't have to be anything fancy, but the structure and responsiblity will help rebuild your self esteem and the salary will start your financial independence. If childcare is an issue remember he has to share the costs of this as after all your being at home is the only reason he is free to have a career.
I've been there, it's so so painful, and sometimes out of dark comes a really bright ending 