Insecurity in my relationship?

Hey, I'm Nic, i'm 15 years old and I've been feeling really insecure lately. I've been feeling insecure about my intelligence, and my talents.. My girlfriend is 14 and she's like one of the smartest girls in her school, and she's a really good gymnast. Me on the other hand, I was unschooled for about 5 years, and I just started an online school about a year ago and all my grades are B's. I just quit Basketball because I felt bored with the sport considering I've been playing it my whole life and a lot of the other kids on the team were better than me so I felt I wasn't going anywhere with it. I recently just started MMA.. I feel like I'm comparing myself to other people too much.. I'm always afraid my girlfriend will leave me because I'm not smart or I'm not good at anything.. It's really annoying and causing a lot of problems. I just want some tips on how to overcome it or some advice or something. It's driving me crazy and my girlfriend.. It hurts her feelings because I'm insecure about her relationship with one of her guy friends.. They're really close and he's also a good gymnast so they kind of share that passion and me and my girlfriend are kinda opposite so it makes me insecure and makes me feel like she'll want a guy more like her that she can relate to. I don't know.. I just need some advice.. Any help is appreciated. Thanks for taking the time to read this :)