This is a really important topic.
The first time I hit this issue, I hit it obsessively and with enormous emotional pain and overwhelm. I was between chemo2 and chemo3, at a training workshop, and a tutor talked about a client of hers (who had severe hormone imbalances that were treated with intensive hormone therapy throughout her adolescence) who got breast cancer at 21 and over the next few years had 51 different cancer treatments for new growth, local and metastatic cancer recurrence. I did not take in that I am nothing like this person, I just confronted the fact that I currently have more reason than before my cancer diagnosis to expect to die within the next couple of years. I went into a total tailspin for about 12 hours.
My Jungian Analyst, who is a Buddhist, told me that many schools of spiritial development suggest that meditating on one's own death can be a valuable process, once it is disconnected from the current trauma that we all experience in getting breast cancer.
So I have found myself obsessed with this issue three or four times since then. I am learning to go there with the goal of learning to accept this idea as a reality, a possible future, but to recognise that it is just a possibility, not a certainty, and that I do not want to waste any of my remaining life in a state of panic and distress about the idea of dying soon. It is not just about wanting to live in this moment, it is about not wanting to waste this moment by filling it with panic and distress. Until I am dead, I still have a life to live, and while the risk of dying is now, thanks to my breast cancer, a part of my present moment, so is the fact that I have a good chance also that I will not die for a few more decades.