From John August, screenwriter (Big Fish, Corpse Bride, Go):
“Writer’s block” is an overused term. When a writer claims to be suffering from it, he is usually wrestling with some combination of three common problems: procrastination, perfectionism, and fear. “Writer’s block” is a romanticized catch-all that distracts from these real issues.
Screenwriters can use a range [of] techniques to get over the hump, from setting a kitchen timer, to breaking work down into manageable chunks, to writing in an order that makes sense for the way you work.
The rest is here, along with an episode of John August and Craig Mazin's Scriptnotes podcast.
I think this is probably true—I've suffered from all three of these problems for as long as I can remember. In my case, the perfectionism (and fear that whatever I create will not meet these impossible standards) leads to procrastination, which later becomes either a last-minute rush job that actually turns out okay (but burns me out) or abandonment.
If only I could have overcome this during my college years instead of continuing to rely on deadlines (self imposed now, and not nearly as scary) to motivate me. But I am trying not to be too hard on myself. August's suggestions here are good, and I'm looking forward to listening to the podcast to see if he has any others.