You feel like sh!t because your ex's actions don't make sense to you. They don't to me either.
As for friends telling you to get over it "after all this time", well, let's just see how they react when their own marriages are flushed down the toilet. It's so easy for the smug marrieds (thank you Bridget Jones) to patronize and give advice. I have to laugh when one of them winds up in the same boat. All of a sudden, they finally "get it".
I have actually had people apologize to me for their stellar "advice" after they have been dumped. They never thought it would happen to them.
My H left Nov 07, and promptly bought a house with OW March 08. Four short months after leaving me. AND WE ARE SILL MARRIED!!
So I know how heartbroken you are. I have been beside myself about it and there's nothing I can do either. Plus, I know it will be even worse when we are finally D'd, and that is coming up in the next month or so.
My H moved in with a single mom too, and the only thing that gives me any solace at all is knowing that being a step-parent isn't easy. And the bitch your ex is with HAS FOUR KIDS??!! Heh.....
If I were you, I'd go as pitch black as you can. I realize it's more difficult because you have kids. So, hang in there sweetie. You and I will be ok. Eventually.