I read the last thread you made and figured it was over then, and that she was obviously cheating, but it was apparent that if you walked away then you would've regretted not trying to work things out so I guess you did the right thing by giving things another shot. At least you can walk away with some closure in knowing that you did what you could to save the marriage.
I don't know all of the details of your marriage, but people do change, so even if you two were right for each other when you first met, it didn't mean it was always going to stay that way, So don't think there was anything you could've done differently to change things, that might not necessarily have been the case. Sort through your emotions as best you can and find a way to cope until you can get over your feelings; try and plan some events/activites for you and your son to do, anything to keep you looking forward and not looking backwards. Try not to let your thoughts linger on what could've been done differently to save the relationship. Also realize that it doesn't matter whose fault it is the marriage ended (both of you probably take some blame, but I can't judge either of you without both sides of the story) because you're both better off without each other, whether you wan't to acknowledge that or not.