Brain Freeze?

Do you ever have a mental block about a project? Sometimes I am very creative, and interested in doing all kinds of things. But give me an assignment, and a curtain falls before my brain. I can think of 50 things I want to do instead. And there are at least as many things I should do before I start. Why is this?
It isn't a new phenomenon. My sisters joke that they should never ask me to make something, but take what I offer them quickly. They know about the difficulty I have when I must do anything, even if it's something I want to make. It feels like my mind has already played with that idea, and it wants to go in a different direction.
Since I have had a certain assignment, I have made 3 little Fragments quilts, 6 rock mosaics, 5 small purses, 6 collaged books, 4 fused glass dishes, a weaving, numerous small sketches, 1 quilt which I adapted to fit the assignment, and 2 starts on the assignment, which I then chose to make not according to directions.
I choose to call this Oppositional Defiance Disorder. I know that is a real diagnosis certain other people suffer from. I suspect it may be behind my feelings of " You can't make me" whenever I am told to follow the rules. It would be nice to benefit from the many fine associations and opportunities that would be available to me, if I could toe the line. But I get a feeling of having to catch my breath, and step away when faced with rules or an authority figure. I have an urge to slip out of the room, become invisible.
So, if you want me to make something, don't ask me to do it, or I never will. And if I made something you like, just like it as it is. Because I might make variations of it, but not if you ask me to.