Oh, Spook, I am so sorry. I have the kind of relationship you describe with my dh and am only too aware how lucky we are and how fragile it all is.
However much you love him you have to make sure he knows how much this hurts you, because if you do forgive him and rebuild things, you need to be sure he 'gets it' and so won't ever do it again.
IMO the only way to sort this out is to talk and talk and talk, get right to the bottom of it. If you do still love him so much, don't be afraid to tell him. He needs there to be something for him to fight for. In the same vein, he also needs to understand how hurt and angry you are, as he needs to face the consequences of his actions, and what he stands to lose.
I would try not to lose my rag and shout irreversible 'get outs' and suchlike. Try to keep your options open and not get backed into corners by your anger and pain.
Obviously, Relate may be helpful. They will get you talking so that you can discover his reasons.
And don't stop posting on mumsnet, you will need a vent for your feelings, someone to rant to and sound out ideas.
Whilst I would not advise letting your dh treat you badly, if he has been a wonderful husband for 10 years it is well worth making every effort to get your marriage back on track.
You have my support, totally, and wish I could give you a big hug right now.