This happened to me last year OP, he left when our baby was 10 weeks old.
He too had met someone else at work, he lied to me about it for months and months, leading me on.
I found out about the other women about 8 weeks ago and he has been moved out for nearly a year.
I didn't know how I was going to get through it honestly, like you I didn't see it coming.
I had some counselling and I am on a low dose of anti depressants.
I never thought I would be happy again, I still have moments where like you I miss the family I always thought we would be. But there is happiness. My baby makes me laugh everyday and I feel so lucky to have her.
I still miss him, but I know I am better off without him. Before I would have done anything to get him back. Now I know, although it hurts, it is the right thing for me and my child.
I know it's annoying to say but time does help, and as my baby has got older it has just got more and more enjoyable and more and more easy.
Those early days are tough, just do what you need to get through the days. I think it takes years to get over a betrayal like this, I wouldn't say I am completely there but I am definitely a lot happier. I can talk about it without crying and I can go out without feeling anxious about someone asking me about him.
PM me if you ever need someone to chat too xxx