The topic that Ivan Ilyich is thinking about here is his approaching death. He is in pain, but he is trying to occupy himself with work so as not to think about the pain he is feeling. This pain leads him to think that he isn't going to live for a long time and that his death is imminent and closer than he thought. He is not ready to die, which is why he is scared and wants to forget about the fact that this is going to happen, whether he likes it or not.
