Lithium doesn't help for psychosis that I am aware of. It might help with mania which helps with psychosis. But if you are experiencing them separately I can't see it helping. I've taken lithium for 7 years almost and it hasn't helped the psychosis at all. Barely helps with my mania either.

I hope the Seroquel helps. I wish I had something more useful to say, besides keep trying and hanging in there. Have you tried zyprexa ? It worked better for psychosis for me than Seroquel.