Her happiness is not within your control. If she is truly happy, she will find it in herself.
Because her relationship with this OM is still fairly new, she is still in the honeymoon phases. Everything about him is bright and shiny right now. But, as you stated, you've been best friends since you met. He doesn't have the history with her or the family. Part of all the newness of relationships is that they have no expectations of the other person (in other words...no responsibilites). This cannot and won't last for long.
You HAVE to leave her alone right now as contrary as it is to every cell in your body. Spend time with your kids, be the responsible dad, and get a life (do something different for yourself, start working out, taking fun classes, jogging, wine tasting, something so that she can see that regardless of what she is doing, your life goes on). It will feel fake for a while, but do it anyway. She'll notice, and you'll feel better.
Keep posting here. There's a lot of great advice.
Me 36 Husband 35 D5 S2 separated: 10/29/07-present Served divorce papers 1/22/09 "When the world gives out beneath your feet, it is time to learn how to fly."
