Practice being confident , and lowkey MENTALLY pretend you’re someone important or someone that has a cool persona.
As a kid I got some of my self esteem from 80’s rappers

Their song content exudes self esteem and they also TAUGHT self esteem.
Rakim said “My self esteem make it seems like the thought took years to build”.
Kane - called himself King Asiatic NOBODY’S EQUAL
KRS-would tell your how smarter his is than everyone aka The Teacher.
LL -LADIES LOVE Cool J
Slick Rick- Jewelry game, drip, swag, and his words in his raps made him above everyone in his mind.
Excelling in sports was the main thing.
Chicks also built my Self esteem as a kid. Being handsome has its perks.
Subtlety Showing off in something you excel in works.
Last but not least, when you get older you just don’t give a damn, and realize that YOU control your self esteem, not people’s feelings about you.