@sparky779j,
I'm really sorry this has happened to you - it's a horrible feeling that hurts; and that hurt doesn't stop. Over time it'll get better, count on it, but it'll never stop hurting completely. So your first task ought to be getting yourself together; absent of that feeling of desperate pain. We tend to turn to jelly at such times, and can act in very bad ways - resist it.
Chances are that the motivation for what she's done has little to do with you, but more to do with herself; problems, issues, discoveries, age, the situations, attractiveness, insecurity, curiosity.. hell, there could be a million reasons. What I suggest here, first off is that not blame yourself; unless she tells you that you otherwise (in which case take it seriously and humbly; no man can improve wrongs he's done when in denial).
I was a cheater once, to unbelievable levels, then it happened to me in a particularly bad way - and in that instant I realized what suffering I'd been party to from the other side and now live with that regret daily. But of the married women I was with, there was often very little wrong with their husbands; nor with their relationships. This was something they had to go through, for one reason or another that was of their own motivation, their own 'issues'.
If you love her, fight for her with patience, honesty and forbearance. Don't get mad, don't get mean. When you hurt, show it, but do so respectfully without imposing yourself. If its within her to right the wrong, heal the pain, she'll be back - as long as you show yourself able to be mature and understanding. If its not within her, no hurt, no imposition, arguing, prompting or pleading will change that. Remember yourself.
Good luck - that's a tough, tough place to be. Be strong for your children