I understand your feelin,
I know how it feels being lean.
If this is familial there nothing much you can do unless and untill the change is from the genetic levels. But do not worry there is till time to gain weight. You just need to do some workout and have balanced diet to gain weight. There isn't a chance.to gain height as you are already 21 your growth spurt is over. You need to hit the gym and ask the trainer for the bulk gaining exercise along with supplements and you can also get you diet chart from your tariner.