Unfortunately our bodies are designed for our natural diet, not some diet we consider politically correct in the 21st Century. Why don't you pack it in with the vegan religion for a while and get some nutrition into yourself? It's hard enough to get from the depleted foods in the stores anyway, without depriving ourselves deliberately. The lady who told you your immune system was weak due to being a vegan is probably right; and she's probably seen lots of similar cases.
There is a lot of crap out there we shouldn't eat, but it's easy to get into an obsessive mentality about it. What we need to eat is even more important than what we need to not eat. Good luck.