Agreed -- it's complicated, time consuming, and/or expensive to be vegan for a long time without developing deficiencies. Especially if you want to avoid soy. I get annoyed when someone who is pale, sickly, tired, and spacey tells me to eat vegan like them because it's healthy (obviously they're not doing it right, or it's not the right thing for them).
You can do rice/pea protein and various supplements but it gets expensive. You're trying to feed the body a diet it wasn't engineered for and have to contort what you consume to make up for that. There's a lot of mis/disinformation in the vegan/vegetarian literature, like how cows and gorillas do fine eating grass and therefore we can get all our protein from vegetables. Yeah right, we lack the second stomachs and/or huge colons that break down cellulose into useable protein, so that doesn't fly.
But as you said, it varies by physiology. I noticed that the most successful vegans/vegetarians tend to be green/blue/hazel eyed people with some blood type other than O. Supposedly they are later evolutions (or engineered products) better designed to handle domesticated grains and such.
About supplements, I found that unlike mainstream (even junk food) diet, an all-natural whole food diet even with meat can lead to certain deficiencies because they're not fortified. Foods today are not grown in soils as micro-nutrient rich as they were a century ago, so getting by on a whole foods diet is harder than it was. The parts normally taken out through processing still aren't as rich in some vitamins and minerals as the fortification powder added to it. Compare the nutrition label of an organic cereal vs cheerios for instance, organic rice versus some mainstream fortified rice.
So in the quest to keep yourself free of toxins, you risk becoming deficient if you don't do it right. Thus you'd have to eat enough and strive for enough variety in your diet. Sometimes people screw up with the simple things like not getting enough fat, which can lead to cardiovascular issues, auto-immune diseases, and hormonal deficiencies.
Ethically I'm against eating meat, but biologically I'm all for it. For what it's worth, I find that pinto beans can stand in for about 50% of animal proteins without harmful effects. Maybe next life I'll be born as an alien that can live off silica jello cubes, but for now I'm in a body that requires real nutrients. If you save a cow by not eating it, but then spend your life as a space cadet who can't think and work hard enough to make a positive difference in the world, then wouldn't that be unethical to throw away a bigger good for the sake of a pettier one? So for me it's about net positive impact.
If it's not genetics or dietary deficiency, then check your salt and sugar/dairy intake (I know you said you don't do dairy much). Especially the salt. Soups, canned food, chips, processed or preserved meats, cheese, bread, and fast food are pretty salty. I don't know how it does it, but it makes hair fall out if you're overloaded with salt. Maybe it clogs up and suffocates the follicles, I don't know. As for the sugar/dairy that can exacerbate candida or other yeast/bacteria that attack hair follicles.I also started to get thinning hair at the age of 19. What's that all about? Protein and zinc deficiency maybe? hmm..