It took me until my 50s to stop being self-conscious about my slender body and skinny arms. Exercising more helped a bit but it was mostly in my head. My advice - love yourself now! (The sign was for a Twitter pro-nudity campaign)
[M 31, 65kg, 175cm] -- Used to be bullied in high school because I was so skinny. Feeling way better with more weight, still feels weird after being so thin.