I agree with the other posters; yes, you're unhealthy and it's time to make some changes. I'd suggest that you get checked for diabetes, which can cause you to not gain weight, and also lightheadedness.
As ducky said, try to fill 1/2 your plate with vegetables at each meal and while you need fruits, limit them, because fruit tends to have a lot of sugar (even natural sugar turns to fat). The best fruits are those with the most fiber, such as apples, pears, berries, etc.
You need to balance a healthy diet with moderate exercise. Walking around outside for 30 minutes/week is like nothing. You need to get adequate exercise to get your heart rate up, keep your muscles built up, etc.
You're young and at 19, you should be able to turn things around. If you continue on your present path, you risk a very wide variety of health issues later in life, such as heart attack, stroke, diabetes, and many more.
YES it is possible to be an "ideal" weight while being very unhealthy.
For example my sister got down to her ideal weight via drug use (Meth). Her hair was straw, her skin waxy, and her eyes glossed over. Her teeth even started rotting. No one in their right mind would have considered her healthy at that point- no matter what her BMI said. An extreme case but point made?
Without eating enough fruits and veggies you aren't getting enough vitamins. That means you're at risk for things like scurvy and other disease caused by malnutrition. It might also explain why you're the smallest of your friends- not enough nutrients= not enough food to grow. Also eating only junk, even if you aren't overweight, can increase your chances of getting diabetes, high cholesterol and heart disease. So yes, you need to change. It's never too late, but the sooner the better (and the less amount of damage).
Try to cut out the junk. Add a couple fruits to your diet, even just an apple or 1/2 a cup of berries with breakfast. Add veggies- try to have half of a dinner plate covered in good fresh veggies like green beans, carrots, broccoli, spinach, salad, etc. (just not all potatoes and canned veg). Make sure you're getting lean proteins, and eating whole grains (all that processed bread has a lot of sugar, fat, and crap). Get lost of fiber, most dr's recommend between 25-30 grams of fiber a day (that's a lot of veggies). Switch to almost all water. You need fluids yes, but 5 glasses of milk a day is a LOT of calories and fat. And can really back you up as far as poos go.
I also suggest you check in with a dr. We tend to know our bodies much better than others and being lightheaded is cause for worry. Ask about getting a physical and tell your doctor about your diet. They might be able to give you pointers or send you to a dietician.
I think you are right to be concerned. I firmly believe that we can tell when our health is taking a downturn. It may not be visible to everyone else, but if you can tell, that is enough. It wouldn't hurt to just have a check up, and voice your concerns with a doctor. Do you have a regular doctor? You're young enough to make these changes, and have very good success.
Bravo for your very good sense! Good health to you!
Tammy