Lithium was really not my medication, despite taking it 6 to 6.5 years total (1 + 5.5). I experienced about 10 side effects from it (oddly, not weight gain) and it would never have worked sufficiently for me for either mania or depression alone. Actually, I think if it did do anything, it was more helpful for my depression. It didn't cause mania, but it didn't curb/extinguish it for me. I think I took Lithium alone (monotherapy) for about 1 day before I was put on an antipsychotic, too. I've been on an antipsychotic ever since (14 years).

For some people Lithium is a miracle worker.

A good combo to consider for depression relief could be Lithium plus Lamictal, or Lithium plus an antipsychotic that is known for depression relief (Abilify, Seroquel, Seroquel XR, Latuda...) Or some people can tolerate an antidepressant with Lithium. Do talk to your psychiatrist about options. Or maybe a little bit more time will bring relief.