I'm going to rant a little because I'm a bit tired of other people's whining. (Not you, of course.)
If you are a good writer, you can write anything.
And when I say anything, I mean anything. A query letter. A play. A screenplay. A poem. A novel. A marketing flyer. A blog. Whatever.
Whenever you switch genres, it's like switching strokes in swimming. The principles are the same for all strokes: you push the water and you move forward. Everyone has a stroke that's strongest, but if you understand the principles of the thing, the physics behind it, the methodology, and you have some basic skills, you can swim anything. With training you get faster, push more water, move more efficiently.
But don't tell me that because you stink at butterfly you can't swim. You just need to practice. Sure you may look all floppy for a little while, but you'll get better. Unless you whine and don't do what you're supposed to do.
To switch back to writing....
Let's say you're writing a novel. You push through. You put your words in. You train. You read and understand the concepts that make up a 'novel'. You finish it.
Congratulations, you've learned and developed one particular, difficult stroke. You've learned to put one word after another. You've learned how to hold a reader's attention and how to braid plot points together. You've done a bazillion little things that have made you a better writer!
And let's say you want to sell that novel. Great! Time to query.
Time to write a letter.
What? You've finished 500 pages of words and you can't write a one page letter?
To this I say: (pardon my French) Bullshit.
You can still put words down, in their correct order, in a way that is both interesting and vivid. You can use spell check, exactly like you did for that other batch of pages that you wrote. You can edit and revise. You can hold a reader's attention and braid plot points together, and you can put one word after another until you're done. Sound familiar?
That's right. It's just a different stroke. You will not drown. You will not fail. Just do what you know how to do.