I can't add too much more. The members above made great points and suggestions. I just want to reemphasize that severity of bipolar illness and number of medications are not necessarily congruent.

I have taken just two medications that were quite effective. I have also been on behemoth cocktails (more than now) in order to handle symptoms. It can depend on many factors, including a person's individual reaction to certain medication(s) in terms of it's effectiveness or side effects. Sometimes something works great for a few years then not as much. Sometimes a medication is a wonderful fit, sometimes not.

I hope what you are on serves you well. Though Lithium does have a clear therapeutic limit to its dose, Seroquel's dose can be worked with without adding a third medication. Your 200 mg is not a particularly high dose at this point. But who knows. Maybe a third or a substitute for one of your current medications may be considered in the future. Or maybe not.