Going last to first, I first considered what I would do differently if my life would end in 6-12 months. First, I would make the best of my time with my family. I would work to finish my dissertation within the year. I think that would be doable if I had such a harsh deadline, to be blunt. I would make my will clear. Push myself to meditate one hour a day. Finally, make sure I don't do anything that hurts my family or makes them think less of me.
For the second thought, OP expands that they were living with 24/7 instant gratification which causes them to feel unsatisfied, without goals, and live a mediocre life.
One comment on this post I found meaningful was someone stating they sought to live a radically optimistic life. They treat the world as if it is what they hope it is. I wonder how this could fit with nihilism. If the world is what we decide it is, then what is our optimistic meaning?
Finally, in response to what I would do with more time I really do not know. I have my hobbies, job, and family. Take these away and I have nothing.