Op you didn’t respond to why yoire trying to cling on to this marriage like this. Maybe you didn’t see the question, but maybe you don’t know?
You know he’s lying to you, you don’t go from loving someone, wanting to marry them to never thinking of them again so fast. But he’s trying not to making it worse again for you
I think examining why you’re staying is key here. I also think there are nuances to this, this affair started before the cancer diagnosis. So what effectively happened is he didn’t stop it due to the diagnosis. He was already looking to exit the marriage, it was clearly an exit affair .
is he getting angry as he feels the cancer is being weaponised against him? It does read like the two are intrinsically linked for you, where as he started this before rhe diagnosis and was not tp know what would happen. So addressing why it started when he thought you were healthy and why he was clearly trying to exit the marriage at that stage is important.
does that make sense? He didn’t start this affair as you had cancer. He started it for another reason and before. He was already having an affair when you were diagnosed. So something was broken at that stage. Understanding that is key
however it seems he is now in a position he just lies to you and says what he thinks you want to hear and he is trying to stop the cancer being linked to it., but obviously failing. So he is not being heard. And although this marriage is already over, if it wasn’t. This would end it. As both of you are deeply unhappy. You lash out. He lies and fakes it. It is a vicarious circle that unless addressed is going to cause much more pain before you are forced to split as neither of you can live like this, him lying and pretending, and you lashing out.