Deciding whether to divorce him

Hello group,

I'm new to this forum and hoping that someone can help me to clarify my thoughts about whether to divorce my husband.  A few weeks ago, I found out about my husband's 3.5 year long sexual affair and numerous other romantic and sexual flirtations or internet relationships (that he claims did not result in actual face-to-face sex).  We've been together for 12 years and married for 6.  We had maybe 8 really wonderful years.  Things between us have been really bad during the last few years and especially the last year.  He's treated me horribly.  He's been defensive, combative, uncommunicative, and nasty.  His ADD problems and sex addiction led to him getting fired from his job and a cascade of other problems.

The one thing that most makes me want to stay with him is the fact that we have a 2 year old together.  I don't want her to experience the trauma of divorce, and I don't want to be a single parent.  I am certain that my husband would have another family within 5 years and will most likely forget about our daughter.  (It's actually the pattern in his family with his father and 2 older brothers, yikes I know.)  I also always wanted a second child but it is hard to imagine a future where that would happen after getting divorced from him.  He is newly aware of his ADD problems and says he really wants to stay together, but at this point his words are essentially meaningless as he's lied to me for so long and his actions do not indicate that he values me or his daughter.

The reasons to leave are that he's been so awful and lied and given me an STD.  He betrayed me and so did his friends.  Entire circles of his and then our friends knew what he was doing and even in some cases assisted him, for example, by providing him a house to have sex at.  So I really feel degraded and humiliated.  I really adored him and idolized him in some ways so the shock that he of all people did this to me leaves me wondering what good there is in the universe.  I thought he was such a good person but the whole time I was thinking that, he's been building what is effectively a secret life to cushion himself from acknowledging parts of himself that he dislikes.  

Looking to the future, he is in therapy and thinks he can become someone worthy of my trust and love again.  But if he wavers on this path, it would be devastating for our family.  OH MY GOD I regret having a child with this man, but at the same time, I really want another baby, if he could be my old husband again who was a good person, I think....

What is best for my daughter?  What is best for me?

Thanks for reading, and thanks in advance to anyone who can help me get some perspective.  I'm really in the depths right now.