I was put on a relatively high dose when manic to bring me down. Once I'd been stable for a few months my pdoc worked with me to very slowly taper off it. I'm not surprised you can't sleep if you are currently on 600 and just miss it. Once off it I was stable on just a mood stabiliser, but occasionally took a small dose of seroquel for a few days if I noticed my sleep getting less and other early manic warning signs.

So yes you can get off it, but do work with your pdoc to do it safely.