I've been through this. i never thought i could be so angry with anyone as i was with my ex. virtually my every thought towards him initially was murderous and full of hurt, rage and anger.
Three years on... he means absolutely nothing to me. infact i feel a bit sorry for him.
The answer is time i'm afraid, but a couple of strategies in the meantime that may help.
Whenever you are talking/texting/emailing him, imagine that a third person (whoever you like) will hear what you are saying. This will help you keep calm and maintain your dignity (which will be so important to you later).
Plan a future for yourself. Even if its just reconciling yourself to life alone with DS. Plan his room yourself, a holiday anything. And try and cope with as much on your own as you can (i had no support from anyone else but hopefully you do have some).
Conjure up a scenario that is worse than the one you are currently in. eg. you might have gone on to have 3 kids together, then found out that all along he'd been having an affair, had a second family that you knew nothing about.. had run up loads of debts, had kicked you out in the street... anything, to make you think that things could be worse. And hang on to that thought.
It wont change whats happened. Nothing can do that, its just about how you cope with it.
And as for him saying you are doing things at your convenience i would say to him politely that he has created this situation, you are the one that is dealing with DS day to day and that it is your right, at the moment, to arrange things for yours and DS's convenience. He obviously wasnt thinking about what was convenient for you when he was sneaking off and having sex with this woman, or when he left to set up home with her.
I hope that wasnt too much of a rant, or too long, and i hope it helps.
If its any consolation, 3 years on I couldnt be happier to be on my own with DS, much better than living a lie and living with someone who could do what he did.