Writers Block

How come I can sit down and write two children's stories in two days, but when it comes down to writing the query letter, I am all thumbs?? I have sat here for over a week trying to figure out how to sell myself as a writer, but my mind is completely blank!! I'm never going to know if I'm good enough to succeed as an author if I can't even get my foot in the door! Any suggestions?

Meditation?

Stand on my head?

I'm Lost! Help!!!