The most common situation for me is to be faced with a list of projects and tasks that are more than I can handle, and doing any one of them means ignoring the others, which are all equally urgent.
I know all the GTD and Pomodoro techniques and everything else, but it's in my head where the problem lies. Once I settle in to one specific task, I can focus and get into the zone to finish it. But that first step—committing to which thing I'll do—is the hardest one. Hours can go by while I'm avoiding the question altogether. I can literally feel the anxiety in my chest whenever I try to pick one and start doing it.