I'm in the same sort of position as you and i have been for a very long time, since i was a young child. I've never been what is considered to be a 'normal, healthy' weight but i'm 17 and still alive so i guess it can't be as bad as everyone is making out. I know being underweight has problems, but i've tried so many things to gain weight but i am anemic and have an unusually high metabolism which means the stuff i eat doesn't get processed in the same way as other people and therefore there's little time to retain much fat in my body. I'm not going to lie and say it's not a problem because i am insecure about the way i look, and it is possible to have an
ED being naturally underweight and wanting to be bigger. I obsess over my food intake due to sheer fear of actually losing weight. It's not a good thing to do. Obessing over anything can happen instantly and it's a difficult cycle to break.
As long as you eat, i.e: your main meals a day plus snacks etc, and you do regular exercise - it just means your body was made to be like this. No one can fault you for that, it's like faulting 'God's work' if you think about it from a religious point of view. Tell other people to back off, that their constant worrying is making YOU worry and therefore stressing you out to the point you don't feel hungry.
I agree about seeing a dietician though, they can help you in regards to your anemia and etc so that you don't have to keep taking iron tablets and stuff. They'll recommend a diet specifically suited to your needs.
Take care x