Man these b*tches are ruthless. 3 kids, and she blasts off with a co-worker. Unbelievable.
But nonetheless, it's the reality. Best advice I can give you right now (and you seem to already have accepted this) is that she's gone and ain't coming back. Once that's internalized, it's much easier to get back.
Like everybody else has said, don't try and get into a competition with her regarding who can move on fast enough. Just stay away from her except for the parenting duties. Unfortunately you also need to accept that there will be frustration there, because you're not going to agree on the parenting/discipline, and one of you will be an enabler.
Getting ass should be far down your list at the moment. Rub one out, go to Vegas, whatever you have to do until your mind clears.