This sounds very familiar. My traditional way of approaching a fest assignment is to write a few thousand words right off the bat. Then, usually a week or two before the thing is due, I decide I hate them all and start over again.

All I can say is that writer's block is like childbirth: the only out of it is to push through it. It may be painful, but at the end of the first draft, you'll have something that seems like a gooey mess but will ultimately clean up nicely and be adorable.