I have had this same issue with switching around being vegetarian/nearly vegan for four years. I, like you, am very thin and have always been that way. I'm hungry more often than everyone else. Sounds familiar?
I still struggle with it because it takes a lot of time and energy to accomplish for me (sometimes I neglect eating too often...), but this is what has helped me:
1. Double whatever on your plate that you think you need. If you think you need about a fistful of rice and beans to satiate you, then eat two fistfuls. EAT MORE. I gorge myself on a regular basis...because I am still usually hungry at the same time everyone else is after doing so.
One time I ate two plates of food for breakfast while the big strong football player type boy sitting next to me (who was not a vegetarian or vegan) only ate one...pretty proud of that.
2. Eat several meals/snacks a day. Again, EAT MORE!! Here is one of my sample menus:
Oatmeal with chia seeds, raisins, walnuts, and almond milk (see how it's not *just* plain oatmeal?)
HUGE smoothie with coconut milk, kale, a banana, protein powder (this is the crucial part for getting more nutrients and calories I think...I use Garden of Life Raw Vegan Protein), strawberries, and 1tbsp coconut oil
(notice the gigantic breakfast?)
This lentil recipe (http://www.food.com/recipe/green-len...jaffrey-438848
) and rice
Small square or two of dark chocolate (75% or higher)
Hummus with pretzels or veggies
Or maybe peanut butter and veggies/crackers
Dinner: Lunch leftovers, or something else with lots of protein and some fats. (falafel with pita, vegan avocado sushi with miso soup and rice...)
Popcorn with coconut oil, nutritional yeast, and salt (recipe: http://www.gimmesomeoven.com/best-bu...nooch-popcorn/
3. Eat lots of fats and protein. Seriously, go wild here.
4. Go to the doctor and get your blood tested. You could have something else going on, or maybe you are lacking in a nutrient. For me, when I first started experimenting with my diet, I was deficient in cholesterol. The doctor told me to eat more eggs, which I did, but in retrospect I think I just wasn't eating enough, period. The cholesterol problem has never shown up again.
5. If all else fails, be vegetarian. It's easier to get more calories more conveniently (eggs, cheese, yogurt, etc) that way. If *that* fails, don't be vegetarian at all. If it's just not working for you, that's ok. Your health comes first. I'm nearly vegan, but I still eat eggs on occasion for this very reason.
The only reason why this is not common sense and is difficult is because the vast majority of society doesn't eat this way so you have to figure it out for yourself. Don't give up too easily, but if it becomes too much you need to listen to your body. Failure to do these things will involve catching a cold every two weeks. :P Good luck!