Welcome to the forum. I can relate to your story. My sister died from ovarian cancer. She lived in the UK and I went to see her. Much of her conversation was about death or how unfair it was she had cancer. I found this difficult at times and kept trying to remind myself I was not the one who had cancer and neither was I the one dying.
It is hard to listen but from a couple of other people in a similar situation it seems a common thing to want to talk constantly about our imminent death once we learn we have a terminal illness. I am sure there are as many different reasons as there are people but I know how difficult it was for me to listen. When my sister passed away I was left with similar emotions to yours. It was hard to accept it had happened even though I knew it would happen one day in the not too far future.
Death is scary for many people and knowing it will happen soon probably makes it harder to accept. As a race we do not talk about death very much and it's usually a more abstract topic. A bit too close for many who have a terminal illness. Please remind yourself we all grieve differently and there can be a long time between the death of someone we love and feeling upset at the loss. For me in regard to my sister, who was close and very much loved, it took at least five years before I had any feelings about it. In retrospect it seems weird and I remember at various times wondering why I did not get as upset as I did when my mom died.
I think my grief for mom and my sister was different in some way. Mothers have such a large role in our lives and when they pass away it leaves a huge hole. It may also be a degree of desensitization as you say because of the person's continual talk. This did not happen with my mom. Her death was unexpected and I had not seen her for a while. She was also in the UK. Perhaps the shock blasted away, so to speak, all my defences against being hurt. I cried continually for months.
I do not intend this to be a comparison or to say you should feel more distressed. I think the circumstances of the death of each of our loved ones determines, to some extent, how we react. Horses for courses if you will. Let your grief come and go as it will. You have done nothing wrong in mourning your mom's death. And you will come to a place of acceptance and peace. That's a promise.