Do you have anxiety? It sounds to me like you might feel overwhelmed with not knowing how to plan out your assignment and so you freeze up. I've definitely experienced this feeling. When I get a big assignment or a lot of assignments, any of which might have some vague instructions or a lot of room for creativity, I get stuck. For me, it's an executive planning problem. I just don't know where to begin and it all just seems so big. I just sit there and go around in circles not knowing what to do, except say over and over in my head, "I don't know what to do". Even though I know what I'm supposed to be doing - I'm supposed to be completing this assignment. But that's easier said than done and it just seems like a huge task before I can break it into less scary, manageable steps.