I'm afraid the bipolar diagnostic is probably right. I would say the recurrence of the depression is a sign of that. Correlation with season variations is the most common, but not the rule.

Medication efficiency and side effect are known to vary individually.

In the end, the only thing that matters is that you find a treatment that helps you to feel and live better. I have heard lithium could help smooth the mood changes. Did you heard of that ? Unfortunately, some people have too bad side effects with it.