10000 lux is quite enough
Do you remember for how long did you use it and how many minutes a day? Oh and you had your eyes opened, right?
20 to 30 minutes a day, while reading magazine or being in the internet with my tablet, for three months. Yes eyes open. And always the same time: 9.00 o'clock. I notised that after a few days I didn't need alarm clock because bright light syncronised my circadian clock. I woke up at eight every morning.
I was also thinking about what you wrote that you feel much better in warmer months when there is a lot of sun. So my idea, maybe it's not just the sun that makes you feel better, but the fact that you spend more time outside. Maybe there is some environmental factor that makes you feel bad, so that when you are out more often you feel better?