You know, it's funny....my psychologist isn't all that impressed with me going vegan either. But she never once mentioned I should quit it....she simply told me to see a nutritionist to examine my diet to make sure I am getting enough of my nutrients, and to talk to my other Dr's about it. I suffer from really bad headaches, so I see all kinds of doctors like neurologist and the sort.
One thing that my specialist said is that when you experience brain injury (which I have from an accident), your brain requires more nutrients, as it heals. The only suggestion she gave me was to eat more frequently and to make certain each meal has enough protein, doesn't matter if it's vegan, vegetarian or omni. A nutritionist told me the same, to eat more frequently. So I try to get loads of nuts in me every chance I get.
Btw, my GP loves the fact that I am vegan. I thought it was because he is one too...but I found out that he's an omni, and just thinks that a vegan diet is the healthiest diet out there...and said that if more ppl were vegan, he'd be out of a job!