You don't have to dig very deeply to find rank pessimism these days. Between reports of ocean acidification and the melting permafrost, it isn't hard to find people who put very low odds on the human race surviving the century. Maybe even the next few decades are a tough bet.
Confronting this stuff, as a single and relatively powerless human being, is paralyzing and depressing. It reminds you that you are not only merely one person among teeming billions (who pays attention to one grain of sand on the beach?), but that talk of taking control of one's life seems a brutal joke in the light of such facts.
On the one hand, it's impossible to function if you think about it. On the other hand, if you don't think about it, then you are making that many more excuses to let things fall to pieces.
One of the reasons I got attracted to Buddhism was because it provided a framework for dealing with overwhelming things like this. When I got past the crappy pop "Buddhism" that was out there, I saw how it dealt with the issue in ways that were both totally pragmatic and totally spiritual at the same time.
The first thing you want to get out of your head, no matter what you've heard before on the subject, is the idea that Buddhism espouses the idea of reincarnation or life after death. There may be some later branches of Buddhism that do, but Zen Buddhism does not (a big part of why I ended up sticking with it). Every responsible person I have heard talk about this stuff has emphasized how all that stuff about reincarnation in the sutras is metaphor. It's a recycling of the vernacular of the day (where reincarnation was widely believed) as a way to make the Buddha's ideas comprehensible to those in the Brahmanic traditions. When you die, you die, and that's it.
But the other thing Buddhism goes on about is the idea that everything you know about existing at all only comes from your limited point of view as a sentient being in this particular space and time. Dying is not a great thing — for one thing, who'll take care of your cats? — but it's also not the whole picture. If you look into it hard enough and thoroughly enough, you start seeing how there are indeed such things as life and death, but that the reality of those things isn't remotely covered by those ideas.
Putting yourself habitually in this frame of mind isn't just some nifty mental exercise. Do it often enough and profoundly enough (read: zazen) and you start to get a little less attached to the idea that you are nothing but this artifact of a moment in time and space. Things are bigger than that. (That doesn't mean anything you can think of is automatically true, though; that's the epitome of self-delusion.)
The other part of this that's important, though, is that you have to arrive at these kinds of understandings entirely on your own, at your own pace. They can't be force-fed to you or imposed on you by some authority, or they become meaningless. A good teacher doesn't do anything for you except show you a few basic ways to think about and deal with the whole thing. Otherwise, they become just another way by which someone is trying to mold society in their own image. Hence why the "state Buddhism" of pre-modern Japan bothered me when I read about it; if you preach to the masses about the inherent nonexistence of phenomena, it becomes very easy to use that dogma as one way to keep them quiescent and cowed.
The last part that's important is that realizing all this stuff isn't an endpoint. It's not something you come to so you can sit there and bask in your spiritual smugness. It makes you realize that all the things you used to be scared of — yes, even things that can spell doom for the human race — aren't that big a deal. You are the universe, and the universe abides.
But that doesn't mean you take those problems lightly; it just means you don't let yourself be paralyzed by them. You and that problem are not even different things anymore; you're both manifestations of something that must, by definition, include both of you. You also realize that the guy next to you, who isn't as lucky as you are to have attained these things, needs all the help he can get to not succumb to the same despair. You see that because you have been looking at all these things through one set of eyes, that your despair is also a product of looking through that one set of eyes, and that despair may not have any more validity than any other prejudice.
I can't emphasize enough how this is not an excuse to be a pollyanna or to engage in self-delusion. The problems facing us collectively are massive and existentially endangering. There may not be enough time left to ameliorate all the damage we've done, or even survive.
But maybe the best way to take that problem as seriously as you can outwardly is to make sure that you don't take it too seriously inwardly. You're a lot bigger than that on the inside. Your job is to prove it, no matter where you are, how you got there, or what's in front of you, be it world destruction or a dying relative or the slightest of frowns on someone's face. Attitude matters.