First of all to note the fear, and work with the fear. Try to have Compassion for the fear. Do some D/D Compassion meditation about the different ages people die. Especially people who are young and perhaps weren't ready either. We may be more ready than they were. Think of someone who dies when they were maybe twenty, or younger. So do some D/D. We realize that many people die at a younger age without any mental development at all.
Taking refuge is also very helpful here. Reflecting on the Dhamma. Reflecting on what we have understood from the Dhamma. It's a very confidence inspiring reflection. Learn to take contentment with what we have already done. Reflecting on our good Kamma, going back in detail. Reflecting on our Generosity, our good generous Kamma. See that we do have a refuge, that we have done some good Kamma. This helps to feel a certain amount of happiness with ourselves, and what we have already done; this helps contentment to arise. Because if we only think that we are not yet enlightened and we have so much more to do, then we are focussing on just our insufficiencies rather than the strengths we have already developed due to the Dhamma. So take refuge.
Reflecting on the Buddha helps us overcome fear. Understanding that we have this potential within ourselves, this helps to give us a refuge. So reflecting on the Buddha and the Dhamma are very helpful with fear of death. Taking contentment with our life up to the present, is something that when people talk about, "How can I help someone who is dying?", I often say, "Talk about their good qualities, talk about what they have done in their life that has been good. Try to remind them of these things, make them happier, and more content with their life thus far." I actually did this with my aunt, who was diagnosed with cancer, and they didn't expect her to be alive for more than four months. I wrote her a letter, it was a basic gratitude letter, reminding her of all the things that she has done that helped me, and how grateful I was, and how wonderful it was that she was able to have such wonderful qualities. I also talked about living each day and being content with it. Apparently this letter had a great effect on her, it changed her mind at that time, and more than a year later she was still alive. It brought a lot of happiness to her, and she started to think about the positive, beneficial things she has done in her life, and feeling contentment with them.
So rather than thinking about what's left to be done, or someone who's dying who may think, "Oh, I have so much more I'd like to do", try to take contentment in what we have already done that was good. Reflecting on one's good Kamma, reflecting on one's generosity, reflecting on the Buddha, Dhamma, Sangha, taking refuge: these are very helpful in dealing with fear of death.