I've just found out that my husband of 14 years with whom I have 3 children (ranging from 13 to 6) has been sending messages on Facebook to a girl who he went to school with. The messages got more and more explicit and now I know what her vagina looks like. 
I was on his Facebook this morning (I got suspicious about the way he was holding his iPad when he was messaging so decided to check his messages and now I wish to God I hadn't, I suppose you get what you deserve when snooping.)
I have no idea what to do, this is a nightmare that I never even imagined that I would have to face. I feel so incredibly sick and heartbroken.
He swears he has never done this before, I am unsure whether to believe him. He's in the military so has plenty of time away where he would have the opportunity to cheat 'properly', IYSWIM.
I just cannot see a way of ever trusting him again, and I cannot see a way past this. I used to think he was the greatest husband, now I don't know what to think.
Help me figure this out please!