The biggest reason for failure in going veg is that people begin to feel bad and blame it on the diet. The fact is, that vegetarians can eat poor diets, just like anyone else. Just not eating meat is only half the idea, the other half is to eat an abundance of:
GOOD oil- vital, do not skip (olive, safflower, flax, but don't cook with flax)
whole grains (brn rice, quinoa, whole wheat/rye/oat, various whole grains & whole grain pastas & cereals)
all the colors availbable in vegetables and fruits
protein - beans, nuts, tofu, seitan, and convenience foods, if you like them, I do
instead of dairy, choose from the many soy/almond, hemp, etc milks on the market. You can also find a variety of cheese products in vegetarian (only contain casein, nothing else) to even rice cheese products.
If you leave out one of these important food groups, you will begin to feel lack luster and lacking, because you are.