I fit the type you're describing, I'm about 5'9" 135 or so. I work out (lift) 3 days a week, and only recently began watching my diet. Before I started working out, I could wolf down a buffet of twinkies daily and never gain a pound, I'm just genetically thin. It's very hard for me to gain weight, both fat and muscle. Nowadays I need to take a couple protein shakes daily to maintain my muscle mass (which I've recently put on and am very proud of

).
That said, I don't feel like I'm anywhere close to having an eating disorder. I generally eat whatever I want, I just stay away from sugars, white flour bread items, and obvious fatty stuff. My portions are generally small to medium not because I'm concerned with weight, I just don't eat to get full. I chalk it up more to having positive self-control than to unreasonable self-image worries. I strongly feel that most people eat way too much.
Am I happy? Well, everyday I wake up in my tiny Calvin Klein's, strip down and hop in the shower, where I completely love myself for a half an hour. Then I put on a small white tee and a good pair of low cut, form fitting jeans. I feel incredibly fucking sexy all the goddamn time, I have men and women checking me out quite often, and I'm a complete fucking stud in bed. I wouldn't say my self-esteem is lacking.
It's my understanding that people come in all shapes and sizes. Some are big, some are skinny, just play the hand that your gene's have dealt you and be happy.