It takes a long time OP. We're 15 months and 2 days (sad I can tell you the date and time my life as it was stopped). He was my best friend, we've been together longer than we lived before meeting each other.
It wasn't sexual, I wouldn't be here if it was. They became friends at work, she is a lot younger (not much older than our oldest), he said he was bored and she was fun, they got the train home a few times, there were lots of emails and eventually the day I found out they'd had their first 'lunch date.' (It had taken them 12 weeks to work up to that) She thought she had the green light to start texting him whilst he was at home. I'd been suspicious for a while, he pulled away from us all, was dismissive of me and I over compensated by being the perfect wife, tried harder to make him happier god I could go on and on.
It's fucking brutal. I lost 2 stone which I could have done without losing, started smoking again and drinking. We went through the hysterical bonding thing and it felt ok but then you crash, it pervades everything. We had a wonderful family day out last weekend and I quietly sobbed in the car on the journey home, I'd been watching all the other families thinking those men wouldn't do this to their wives, why me?
You are very brave to try to stay together, I'm not sure I would in your position, but he was my best friend, it's so hard to walk away, especially with a tiny baby. No one knows you or your husband, no one will know if he will do it again (they don't always do it again) but I understand why you have to give him the benefit of the doubt to hope it'll work and he won't do it again. I sort of felt it's better the Devi you know but by god did I really know him?
Make sure you have support, don't feel as though you can't tell family and friends but be aware that once it's out there you can't get it back. He has to completely cut contact, block her on everything and if there's even a sniff of any contact he's out, no discussions, no arguments, he needs to put you and your child first, if his feelings for her means he can't do that then it's probably gone too far.
It's just very very hard, a long slog and it never goes away. You never have the peace you did before you found out.
Good luck.