I hate that kind of thing too. I understand your feeling exactly. I'm much older now but when I was high school/college, I got words like this a lot.
Even today in my mid 30s, my parents don't talk to me like this anymore because they know I am successful now. But my relatives who haven't seen me in a long time say things like that. Even though I am very successful, just because I don't have X or Y or haven't done something they think is important, or they think I'm still really young for some reason (even though obviously 30s is no longer college aged), they say this to me. Like what the hell am i supposed to do?
But anyway, because I have a lot of confidence, I don't even care what they say anymore. I know I am better than them. So that's how you beat it. Because this problem might never go away.
Well I hate to say you're too young but if you're 18 don't worry too much yet. You have a lot of time and just move forward at your own pace and you'll be okay. You're already having plans to prove them wrong, just be patient.
All the stuff you have accomplished that you said here, I had not done anything remotely close to that when I was your age. No relationships, definitely couldn't try to sing in a band, etc. That's impressive, really I think you will have beaten your problems sooner than you think.