I echo everything ptumbi has said OP.
I cried at CAB when I had to say the words out loud for the first time, because it made it real. It is natural to feel as you do.
But you must, must, must start to understand that he chose this path, he is not the man you thought he was, and he deserves not an ounce of your pity or understanding.
You are obviously lovely. You are grieving, it is natural. But please take him off that pedestal right now because he is a cheat, he is selfish, self absorbed and weak.
He has successfully guilt tripped you into blaming yourself, which shows how damaged he is.
Take time, as long as you need, to build a new future for you and the kids. You deserve so much better than him.
One day you will meet someone worthy of you. This guy has damaged your past, please don't let him spoil your future.
Think of a time before you met him when you were happy. He wasn't in your life then, but you were still happy...so that means there is happiness ahead of you.
Hope the GP helped, small steps, keep posting x