self confidence

I've struggled with weight my entire life. and when I say that people are always like but why your so tiny don't think your fat. when I say I struggle with weight, I struggle gaining weight. I can't gain weight. none of these medicines that everyone claims made them gain 50 lbs. like birth control the shot. or zoloft. none of it makes me gain weight. but I can feel my body getting bigger. like my butt. my stretch marks are so bad on my legs and then there's self harm scars. and I don't have a clue on how to gain weight. i eat like a pig. most of the time.