Your post just gave me a personal "aha!" and even though I'm a GTD beginner, I'm going to leap in with my $.02.
Sometimes I've found myself in exactly the same spot you describe, and I finally realized I've imposed a conflict upon myself. I've tried to do two things at once: complete a specific task, and complete that task within an arbitrarily limited time period. I don't really know how long a lot of things take, especially when I'm doing pieces of them for the first time. How can I fairly expect to get those things done within a rigidly constrained timeframe? A long time ago, I was working on a business launch with someone who'd give me a critical piece of the project with the guideline that "it should take only X days/hours" to complete. Never having done it before himself, my partner had no earthly idea how long it would really take, and his completely unrealistic (and always inaccurate) estimates reflected only his lack of respect for my time and the importance of critical minutiae that were beyond his skills. He knew I'd get the job done, even if it took me ten, 15 times as long as he'd calculated. I internalized a great deal of unnecessary stress and anxiety in trying to meet impossible timeframes, and this happened over and over again. I was lucky--I decided to leave the project when the lunacy didn't abate, and went on to other work. Had I stayed, I'd probably have internalized this bad management practice more deeply in my personal standards, and split my time on task between working and self-flagellating about that fact that I wasn't getting enough done.
I now try to do one thing or another: Either work by time on a task and define success as having spent X minutes or hours on it, or work by task over time, and define success as having completed that task to the NA, whenever that point is arrived. I have to hang onto the successes because everything takes shockingly longer than I'd ever have anticipated, but even if it's in slo mo, things are getting done. I'm aware of the maxim that tasks expand to fit the time alotted, so I'm not mindless about the clock. I track the time it takes to accomplish things to get a better sense of how much I can get done in a given hour or day, but I've found that it works much better for my morale and accomplishment continuity to say "I'm going to do an hour on X" instead of "I'm going to get X done within an hour." It doesn't net me the internal drama, heroics, and demonstrations of performance prowess I used to get from meeting insane self-imposed deadlines, but that's a loss I can live with. I do find I'm acquiring a taste for opera, though.
