I think that's me as well Katie. I'm a very left brain type of person. Like with you, I can imagine up something awesome, like a cool character, or action scene, or even a cool story, but when I go to put the pen to paper, I end up stopping after everything I do and thinking that it doesn't look right, or the story needs to have more scientific backing.
Great example: Right now I'm in the middle/beginning of writing a scifi adventure story, however, I keep getting hung up on things like, how would the spacecraft travel between star systems, and I end up spending all my time inventing theoretical space travel, then I don't want to write it down in case it actually works that way. Or when I try drawing, I hate using the circles and stuff because I don't like seeing that incomplete progress that still doesn't look like what I want it to till I'm done. UGH!