How can you actually STAY a vegetarian without getting sick?

I've tried multiple times to be a vegetarian (which you'd think would be very easy in a pro-vegetarian/vegan, hippie town), yet every time I do I find myself getting rather sick and lethargic after a week or week and a half. I can't stand "complicated" foods with a lot of seasonings or any sauces because I have a  very sensitive stomach, and would prefer to eat more "naturally" anyhow (the closest to raw, if not raw, as I can get).

But how do people do this without getting sick (and without taking tons of supplements...I watched one vegetarian eat breakfast once; a small cup of yogurt and 5 different vitamin supplements. I'd rather not, thanks), and stay away from eating a bunch of pastas/breads? How can I actually STICK to a vegetarian diet?
When I try to switch, I don't change my diet too much. Just try to add more beans/nuts and take out the meat. I normally eat a lot of fruits/vegetables and have no stomach problems from it (even with some of them being "raw", I guess you could say).

I have no idea what the problem is.

There's no magical nutrient found only in animal products, let alone only in meat.

I'm a *vegan*, and I regularly-enjoy casseroles, pizza, veggie wraps, chips and salsa, pasta...
you get the picture.

If you have a very sensitive stomach, and become ill when switching to vegetarianism,
I would suggest that the culprit is *not* vegetarianism, but in fact your individual choices-
aggravating a food allergy or inflammation of your digestive tract, perhaps?

I have no experience with raw diets, but I wish you the best of luck!

EDIT: Nut allergies are very common- have you had a clinical workup or done much one at a time elimination/introduction of foods to narrow down what might be causing it?