@Trudeau25 - I'm in a similar situation (although I was a problem drinker which caused his unhappiness so I am carrying a lot of guilt). H said there was no OW but there was - he "only" had EA until he left then it was physical so in his head, it wasn't an affair.
There is not always an OW but the vast majority of the time, men stay in relationships when they are unhappy - men are socialised in a completely different way to women and can therefore compartmentalise their lives - so for example, most men will happily have sex with their partner even if they don't love them, they are annoyed with them etc. women are socialised to equate sex with love therefore are less likely to do that. so most men will not leave a relationship, even if it is not making them happy, until they have a better offer. that is the reality. so maybe OW was not the cause of him leaving but gave him an exit affair opportunity. To be brutally honest, I was glad to find out when I was feeling really bad - if you are already in the shit, its best to have a bit more then rather than get out of it and then be dragged back in at a later date when you were starting to feel better again - well that was my rationale.
In terms of coping - 100% agree with other posters saying don't make any rash decisions. I'm 4 months down the line and I nearly bought a flat one month after he'd left - I'm so glad I didn't now as he is still paying for our current house which is a great help financially.
I put myself out there with friends, even those where the friendship had slipped a little, and they have been amazing. Keep your days busy - I remember driving home the long way all the time so I would get home later and not have too long an evening stretched before me.
My DC are young adults but I totally over compensated, again partly to fill my time/the void. I did all the house hold chores and let them out of doing anything for themselves - tided their rooms, did all their washing and ironing etc. Don't do this!!! The time will come (I am there now) when you won't feel the need to fill every waking hour with "stuff" and it is hard getting mine to do their own chores again.
Make lists of all the things that were negative about your relationship to give you some perspective. Think of the things you can do now that you couldn't then. We had a very strict routine about dinner and I love being more relaxed about that now. It helps stop the rose tinted glasses perspective.