I have read that getting dental work is the perfect time to fast, because you won't be enjoying the food much, anyway :)
Muscle mass - well if you're juice fasting, you should also be exercising (yoga, stretching and walking are great exercise on a fast) so you shouldn't lose muscle mass.
However, the other questions I don't consider myself expert enough to answer. If you "don't enjoy food anymore" ever, just be careful - make sure you're fasting for the right reasons. I think unprocessed grains are very, very good for you, and I'm not sure what you've read about them being "bad"
If you're nervous and depressed, treat yourself extra nicely! MY significant other gets occaisonal anxiety attacks. You want to begin a fast when you're feeling strong enough to fulfill all of the demands the fast requires. So if your body tells you it's OK, you can count on me, I'm ready to heal, then you're ready to fast. But if you're feeling your body is all screwed up and isn't sending messages that make sense, maybe now's not the best time to fast, because you have to be able to trust the messages you get from your body.