First thing is that have you experienced a mixed episode after being on the doctor-approved dosage of lithium and quetiapine?

Antipsychotics such as quetaipine should be an excellent choice for mixed episodes and did work well for me.
Lithium is still very good, but there are alternatives if it does not work for you.

You see, to be on quite a high dosage on quetiapine; the lithium has the potential to go higher, but a doctor would know what to do with your history and whether to increase it or try something else.