Unfortunately I, too, know this feeling all to well. I pressure myself and berate myself for not starting. But, then I realize that if it wasn't for the 'last minute' I wouldn't get anything done.
One of the reasons that I do this (and I can hardly believe it myself) is that I am a perfectionist and I am afraid I will fail my tasks. This is not me talking, but my therapist explaining to me why I only begin projects at the last minute. She says that when I do things at the last minute, I can push past perfectionism and concentrate on getting it done because I have to.
I never thought of myself of a perfectionist because I have the attitude "if you can't see it from an airplane..." then your mistakes don't exist. On the other hand, when I have the time to do something I try to do it right which means I do a lot of reverse quilting (or whatever task I am doing). With that explantion, I can see that I am a perfectionist. Additionally, if I do something at the last minute and it doesn't turn out right, I always have the excuse that, 'well, I just threw it together at the last minute. If I had more time I could have done it better.'
Oh, my gosh, I just proved my therapist right.