I think the only question you need to ask yourself is: could you trust your husband again? It sounds like you've had reasons not to trust him even before you found it, if you believe it he would have continued if she hadn't got pregnant. Unless you really believe he can earn your trust back you'll be hurting yourself and your children by staying with him.
To be honest, I couldn't stay with someone who refused to take responsibility for his child, but I don't know the situation and only you can know if this will end the relationship. You feel that part of you has died because, in a way, part of you has. You're trust in your husband, the relationship you thought you had, and possibly the future you were planning have gone, so it's understandable that you feel the way that you do. I think it's important to acknowledge and accept how you feel, and to do what's best for you.
