I'm wondering why so many vegans seem to flourish if that is the case.
Do they? I've never seen any actual research that shows they do better than omnivors, just books, forums and websites by converts. Some of them contained as many falacies, cherry picked statistics and straw men as your average Watchtower.
Watch it with young girls too, many have used veganism as a cover for anorexia.
People will often experience a short term lift when they try a new diet that convinces them they are on the right path, but it might not last.
It can really become a problem if the vegan takes up the lifestyle for philosophical reasons and develops an aversion to meat. Not everyone thrives on the diet, even if they are careful and do their homework.
If a practitioner doesn't thrive, there is a lot of incentive to spank themselves for not being strict enough, rather than to admit that their gurus might be wrong, especially if they have been doing a lot of preaching for their new found lifestyle. What can happen then is that the little bit of 'cheating', that helped them do ok for as long as they did, gets the blame for their failure to thrive and they cut that out.
If you don't do well, admit it to your friends and converts and move on.
Chris