My heart really goes out to you MrsWorrier (hugs) Not only has your husband's actions come as a terrible shock, but it has forced you to make a MASSIVE life adjustment. And whereas a cheated on partner can at least let off steam by talking to every day people, because of the sensitive nature of thing,s and your DC's, you are forced to keep everything a secret. That can be profoundly isolating. You and your DC's are victims of his abusive actions too.
As has been said it's O.K to still feel love for your husband, whilst hating his actions. It's better to admit that than try to fight it. What's important is that you don't give in to those feelings in the sense of trivialising his actions or standing by him, as he could be a potential danger to your children. Use your love for your children to counteract your feelings for your husband.
'I just can't believe he was capable of this'
People with such desires have to compartmentalise their lives. They are perfectly capable of being kind, loving, good even, whilst engaging in dark, sordid and twisted behaviour. My Step Dad appeared a lovely man to everyone he met. I even had school friends tell me they wished their SP were as nice as mine. Yet he sexually abused me.
'Where did it all go wrong?'
Not really worth you going insane trying to work this out. It could be he has always had these twisted desires, even before he met you. It is possible that he was a fairly average guy who started watching adult porn, and then gradually got into watching porn with younger and younger girls/children. Either way his actions have nothing to do with you. None of it is your fault. It's not about how much sex you gave/or didn't give him etc, and its definately not the first thing you would suspect.
It terms of moving on, I think you need to go through the same steps someone in a breakup has to go through. After the shock has worn off a bit more, start looking at your relationship. No relationship is perfect. What can you do now that you're single that you couldn't do before? Write a list. What were his other flaws etc. Do you have any hobbies you can indulge in now?
Unfortunately most of this requires time before things get any better. After a while you may start to feel angry and that will be good. I suggest regular one on one counselling to work through your feelings.
Lots of hugs and best wishes. Sorry this is so long.