Yup totally understand 15 years of relationship. Take your time to come to terms with his selfish behaviour, take your time to mourn the loss of your relationship and accept that he has moved on emotionally with someone else, then learn to love yourself more by taking good care of yourself (who knows, you might get to know someone else in future?) Ultimately, you can't clap with one hand. It's understandle that it's tough to give up after so many years, but what can you do if the other party has already given up? The next you can do is heal yourself from the betrayal and on practical side, secure yourself financially and move on.
Any idea why your child is afraid of your hubby only? It's tough to bring up a child. Btw, I don't think it's about "afraid". To let your child listen, I think it's more like setting example and being reasonable. He is 10, yes he is afraid of your hubby now. Do you think he will be afraid of your hubby when he is 20? Does your child know about his daddy's affair? Maybe he is stubborn or rebellious because he is unhappy, as he can sense that his parents are unhappy and not loving? Kids can feel it when their parents are not loving and they don't directly say out their unhappiness and may present it with attitude problems. Maybe he only listens to the dad because dad is bigger size, dad is fierce, dad will physically punish him? If that's the case, it's not going to work in the Long term also. But I do suggest that you should work on your child. Show him lotsa love and provide him lotsa assurance that you love him and will protect him all the way. Don't bad mouth your hubby and explain to him that daddy has someone else (provided your child already knew about the daddy affair) and mummy has to be separated but mummy is always here with you and daddy is still your daddy etc.
Yes you have a lot of things to consider. But definitely dragging on when your hubby has chosen the affair is not a wise choice. Are you financially independent? Is your hubby earning enough to be able to pay you alimony and sufficient maintenance? Do you have good family support? How Long have you discovered the affair and do you have proofs? The way to go about divorcing depends on all these questions above.
And because your hubby is selfish right (he is afraid of his reputation and thus hold on to this marriage, uncaring about your hurt and yet wants to continue his romance), who knows. Maybe after you finally decide to divorce, he "wakes up" and realise that he can't have the best of both worlds and his romance bubble will burst!