shouted at 10 year old

[wanttobeperfect]

My 10 year old son has had a cough for 2 years. Part of it is asthma but there is also a pyscological part too.
I have been very concious that I might cause him stress and try to stop my temper flaring.( We have had years of stress after the divorce etc)
This morning I shouted when he was taking ages getting ready for school and he then shouted back at me " maybe thats why I cough because you always shout at me" I was devastated as we had had a lovely calm weekend. When I asked him when I had shouted at the weekend, he was able to come up with one instant when he had gone off in a sulk after falling out with friends. I was so hurt that I did the worst thing I shouted
" well go and live with your father then" I am mortified that I said that, and how I make everything better again.
thanks for reading this and I would welcome advise