Hi Clarinetgigirl
I reckon that it is easy to overpractise, or possibly better to say "over-work" pieces, yes. I find that sometimes (esp. with exam pieces) one can get it just perfect, and then it will suddenly seem to fall apart. I think that this is due to too much time being spent on it, and possibly knowing it so well that one doesn't really concentrate/think about things well enough, and if you do concentrate, it might be tooo hard!!
When this happens for me, i just give the piece, say, a week's break, and often when i come back to it it will be good again, and i might find that i can learn something new from it!
I often find that this happens just a few weeks before the exam, which gets a little unnerving when you leave it (!) but pays off in the end - leave the best performance for the exam! if you leave it, come back and are still playing well, it obviously shows you know it well and should be able to do well in the exam.
Hope all that writing helps: seems rather a lot to me...(i have a habit of writing a lot

)
I'll shut up now
Piello
Oh, and good luck for your exam!!