Why am I sleepy and depressed all the time?

I dont understand it. I have been to a lot of doctors and got diagnosed with Obstructive Sleep Apnea, depression symptoms and B12 deficiency. While this might seem like its an answer in itself why do Iget the feeling there is a lot more to it than simply this...Any suggestions please shoot. I desperately need some. How serious can B12 deficiency if you dont have pernicious anaemia?