Very insecure with how I look..

I got bullied in high school because of how skinny I was. mind you I’m. A big eater. I eat everything . now that I’m 24 I only weigh 120 and I feel still skinny and I don’t love how I look. I eat a lot and I gain NOTHING. I just want to know if something is wrong with me :/ I WORK at a school and the kids say I have skinny legs 😒 I’m trying to build muscle but nothing works. How do i get bigger legs??? and gain weight ????? I think I have a high metabolism too 😔😔😔😔i wannna be fat.