Look at what I can do

I love to write stories but I dislike writing papers for school. When I write my own stories I feel like I get my idea across so much easier. However, when I am in a class and I get a paper assigned to me, I get a total writers block. I lose all my ideas, I can not figure out how to word my thoughts properly, and I get frantic. I get throught all of that eventually, but then comes the self-editing. I can read over and over and over my paper and not find anything wrong with it but the second someone else reads it, errors come after errors. I always wonder why that happpens?
BUT I figured it out.. its because I dont kno what I am supposed to be looking for. I guess there are specific details I should have looked for throughout my writing that I completley just overlooked. I should have first looked at my papers length.. that is the first way that you can easily tell if the paper looks like enough detail. The other major part that I forget about it the focus of my writing.. it is so hard for me to maintain my audience.. I always feel like I bounce back and forth between how I explain some details. And then there is revising.. it takes forever but its the final step before your done with the writing. I like to have multiple people read my work to see if they can find something that doesnt sound just right.. so I can fix it before my professor takes points off for it.