Girl, if I ever met the people insulting you, there would be trouble 
I’m sure you’re absolutely perfect the way you are. Please don’t let what other people say get you down or turn you into someone you’re not. Seriously, they’re not worth it and what they think or say about you won’t matter in a little while.
But I totally understand the struggle of being so underweight. I‘ve been severely underweight for 7 years now and just can’t seem to gain any back on, no matter what I eat or do. I don’t even like taking pictures of myself anymore because I hate seeing my skinny arms and shoulders 
It’s really a tough situation and I feel like no one else truly understands it until they’re actually dealing with it. So, all I can say is keep being you, girl but please do try to stay as healthy as you possibly can. Don’t let anyone else’s opinions bring you down either.