I didn't find out about my husband's affair until it was well and truly over. how the hell should I react?

Wow. You must really love this man to still be with him. Well sweety, despite all the ups and downs you have gone through in your marriage you made a decision to stick it out with him. I commend you for fighting for your marriage and you can fight as hard as you can but if he doesn't fight with you to keep this marriage moving forward in a positive way, you are going to end up wasting many years in your life that you can not get back. He has to own up to his faults and take responsibility for why you are now so broken. I suggest that you both seek marriage counseling to make this marriage work. There is no way you are going to heal and resolve and be at peace if you don't. If he loves you the way that he says he does then he has to understand why you are so hurt. He shitted on you in a major way. Let him know in order to really put the past behind you and have some closure in order to move forward with him, then you both need marriage counseling. If he is unwilling to do so, then i say start thinking about a divorce. There is no way you are going to live a happy life being with a man that hurt and betrayed you the way that he did with out marriage counseling. If he loves you then he will do what ever needs to be done to repair your marriage. If he doesn't, then he doesn't deserve you and there is a good man out there that does deserve you. Good luck.