I know this might be hard to take- BUT your husband did not actually have an affair- although some people might say that you don't actually have to have sex to be unfaithful.
BUT would it help if you saw this as a silly flirtation that got out of hand?
Think what he didn't do- he didn't pursue this or have sex- that must mean something to you that he held back.
We all do silly things - this seems to have been a flash in the pan. Instead oflooking back, have you talked to him about why he needed this secret relationship? What is missing from his life that he needed it? By that, I don't mean there is anything wrong with you, but that maybe he needs to look at his life and get out of a rut- to me seems like a form of escapism.
Have you thought about counselling- either for both of you as a couple, or you on your own, or him on his own - to explore why he went down that road in the first place.
Along with the others, I agree youshould change your GP- and maybe ask for support in quitting smoking if you want that- there is so much help out there for that now- if you REALLY want to give up, then you will.
Although this episode in your life is stressful, look on it as an opportunity to start again, build a closer relationship with your partner, and give yourself some another chance.