It's not good to mislead your doctor like that, why do you feel you need to hide it from him? Furthermore, low dose lithium is still significantly better then no lithium, how do you feel about going back on a lower dose?

Also, have you had your vitamin D3 level checked? One thing that can cause SAD is low vitamin D3 status. If you want to try vitamin D3 you need to remember it has a half-life of 21 days and also that it takes the body over 10 days to process it into its active metabolite calcitriol. This means it takes about 4 months before it reaches peak levels in the body. Therefore, if you want to try vitamin D3 supplementation you need to take a loading dose. The loading dose required to reach the serum 25-OHD3 target level of 75 nmol/l can be calculated as follows: dose (IU)=40×(75−serum 25-OHD3)×body weight in kilograms. If you don't know your current serum 25-OHD3 level use 25. Assuming you weight 70 kg (155 pounds) you would want to take 140,000 IU. I would spread this out over one week, i.e. 20,000 IU a day for 7 days. Without knowing more about you and your current serum levels I'm not sure what maintenance dose you should take, but a good starting point is 5,000 IU every other day. This is the brand that I take: Amazon.com: Doctor's Best Vitamin D3 5000iu Soft-gels, 360-Count