With regards to mortality as a motivator, I often find the opposite to be true. When I am reminded of death or illness, I usually end up falling into a spiral of procrastination -- anything to avoid the thought of oblivion. I find it to be quite a major source of time wastage and I suspect I'm not alone in this.
When Asimov was unwell following bypass surgery, James Randi wrote that Asimov fell into a depression and gave up writing altogether [1], presumably for the same reason.
In my younger and more foolish days, and inspired by Steve Jobs, I tried to literally live life as if every day were my last in an effort to be productive. I discovered that if it were my last day on earth, I would much rather spend the time joking around with friends or family rather than working, and I fell into the same funk you describe.
If anything, I discovered what my priorities in life were. But I could have done without the moodiness, procrastination, and cognitive dissonance.
While I've never tried to live like that, I know that I'd act much the same... Without the thought that I'm preparing for a better future, my choice would be to have a better present. Every time.
It's worth noting that Asimov acquired AIDS from a blood transfusion during that bypass surgery; he hid it per his doctors' recommendation, so it wasn't revealed until ten years after his death. I'd expect that secret probably didn't help his mood, either.
Martial arts helps me. It builds discipline and gives me more energy that cascades into the rest of my life, and it's fun just on its own. The structure, community and obligation of it makes it more powerful than say, going bouldering on my own for an equivalent amount of time each week.
I spent so much of my life living in my mind without realizing my mind is part of my body and the whole thing needs taking care of and improving.
Set goals and go about pursuing them. It requires discipline but once you surpass the initial blocks it becomes second nature. Also, don't forget of yourself. Do some form of physical activity and set some goals for that too. Rinse and repeat.
Regarding goals, I've personally found them to be counterproductive unless they're simply achieved and a matter of habit. For instance, "go to the gym and do nothing" became "go to the gym and walk on the treadmill" when I became habituated to showing up at the gym. Now I'm lifting regularly and I feel off if I don't get a chance to go work out.
Audacious goals, on the other hand, are demotivating because there's too much chance of failure. If I hold myself to lifting certain weights, I may even end up getting injured. But now I'm happy simply showing up, doing what I have the capacity for, whether it's walking, stretching, or lifting. I can (and will) come back if I have an off day.
Scott Adams talks about something similar in Goals vs Systems[1]. A goal of writing a book is much harder to achieve than a goal of typing a little bit in the morning. Ironically, the latter may be the best route to the former...as long as you don't think about it too much :)
When Asimov was unwell following bypass surgery, James Randi wrote that Asimov fell into a depression and gave up writing altogether [1], presumably for the same reason.
[1] http://www.skepticfiles.org/atheist/asimovob.htm