I am convinced that there is a link between breast cancer and stress so all this drama cannot help me
No, it cannot be helping you.
That is exactly why I focused upthread on the stress management stuff. And I still urge you to put a proper support plan in place around this.
One thing to bear in mind, which I swear is true, is that the affair is nothng to do with you at all. Please do not waste time worrying about whether he would have done this if circumstances or you had been different. He would still have done it. He is having his mid life crisis fun, and he would have anyway, regardless of whether you were the proverbial angel in the kitchen, bedroom and nursery, and in perfect health.
It is simply him seeing if he 'still has it', believing he wouldnt get caught, and wanting his ego stroked, and adrenaline pumped, by someone in that way one only gets in the first weeks of a new fling.
So, if you think logically about it, you could feel sorry for her. I did with my h's Ow (plural), innocent, messed up twits that they were. They meant absolutely nothing to him. It was all about him.