It is unlikely that Lithium would help with the migraines. Nor would it hurt - it is just unlikely to help. What are you taking for migraine prevention right now?

Your story is rather sad and rather representative - Lithium is the last medication to try whereas it really should be the first to try (and, if necessary, discard, but at least try first) since it remains the gold standard of bipolar treatment. There is no money in Lithium, for one, and, many pdocs prefer to prescribe newer drugs that they keep reading about. Probably that explains why you are going to try it a whole 8 years after being diagnosed.