Amanda,
I don't think this sh*t can give you any answers. He just thought he could have his cake and eat it, take advantage of the situation.
You say that he and your daughter are close. That makes what he has done worse,he hasn't only cheated on you, but has betrayed your little girl, and she is too young to be treated like that (but then aren't we all).
Don't try to find out why he did it. He did it because he could. Walk away, break all contact with him NOW, today if you can. If you have left any stuff at his house, mail him and ask him to send it. Send anything of his back without any message.
Cry, cry and cry somemore. Then tell your dd that he has had to go away, and won't be able to see her for a while.
Put your chin up, make yourself smile (bloody hard I know) and say
"I'm beautiful, I'm bright, and I deserve better".
Lots of love to you and dd.
You'll find the right one someday, but honey, HE ain't it!!!