Skin and Bones

So I'm 15 years old and I'm completely skin and bones. I've had 2 open heart surgeries and that has been a cause of me being stick skinny and I also have quite a fast metabolism. People tell me I should embrace being skinny while it lasts but sometimes it's just so annoying not being able to fit into a pair of jeans with mile long legs that are the width of a child's. Any suggestions?  
(Little full body of me ignore my face in the photo though 😂)