Today at work, I attended an interesting seminar by Lee Silber and Andrew Chapman on simplifying and prioritizing based on what they call the 90/10 rule. It can be applied to life or work. Basically, it's a way to focus on the 10% of what you do that you love the most and is most beneficial to you.
As I listened to them, I was reminded of what Mark Tutssell, then ECD at Leo Burnett, once told me. It was maybe the most liberating, stress-reducing thing I'd ever been told by a creative director. He asked what I was working on, and when I told him it was just some crap for one of our crappier clients, he said, "Get it off your desk. It's not an opportunity. Spend your time on opportunities."
Real opportunities are about 10% of what we work on in our business (if we're lucky). The rest is just time-eating stuff. Your goal should be to increase that 10%. This isn't to contradict what I've said before, that you should look at everything as an opportunity when you start concepting. But when it becomes clear that a project isn't going to end up great and has gone past the point of no return, get it off your desk. Do your best to make it not suck, but don't get sucked into the trap of spending tons of your time on it. Polishing a turd, some people call it.
Some projects will never be opportunities. Some projects have potential but get so overburdened with junk that they cease being an opportunity. Once you recognize a project has gotten to this point, get it off your desk.