You're not an idiot, but she thinks that she is better than you, which is why she's leaving.
Use that as motivation. You are the better one. You are the faithful one. Remember that she is the cheater and know that you can only work on you and that you deserve better than that.
Set some boundaries with her, too. Do this to protect yourself and get back some control over your life. If she's being snarky with you, you make it very clear that NO ONE talks to you like that, including her, and that there is a consequence for it.
Another thing: Document your time with the kiddos and document well. You may need it...
