Dealing with this is a process, pain, anger, disbelief... there's no denying it, no quick fix. It helps to read through the acknowledged stages of grieving, as that's what you're doing, grieving.
It's common to cycle through all the stages, and more than once, in no particular order.
Certainly, keeping busy helps, but nothing will actually take it away, time will definitely lessen it though.
I promise, you will feel better than you do right now, but it takes time.
As for your WW.... please, for your own mental health, and that of your children, have as little contact with her as you possibly can. You can't fix crazy, and what she's been doing is just that, crazy. She's also attracting more crazy around her (OM's comments for example).
Look what she's just manipulated you into doing, lying, tricking and deceiving... you've been pulled into doing the same crazy stuff that she does.
You're better than that.
The only reason for contact should be joint finances, if your children aren't also her children, then she doesn't need to be informed of anything to do with them. Protect your kids, who knows what she'll fill their heads with.
Be prepared for her to try and keep contact, keep you on the hook as her Plan B, her fantasy world with OM will explode, nothing surer. Please don't let this happen, you don't need more pain, more destruction.
Her relationship with him is based on lies and deceit, nothing solid. Once their "secret" isn't a secret any more, and they realize they're just two messed up people with nothing in common, the attraction will fade fast
I know you're hurting, I know this isn't how you thought things would turn out, this stuff is never easy, but this is what you have to deal with and I'm sorry you find yourself here.
It's time for you to indulge in self-care. The best thing right now is to be as close to NC as is humanly possible. Your WW is on a downward spiral, don't let her drag you down too, it's a long way back up.
Take care of yourself man, start making smart decisions, NC NC NC... give yourself the opportunity to process all that's happened, without her adding more mess, and make a plan for you and the kids going forward.