I'm 18 this year and i think i am extremely self-absorbed. I dont know really know how this began, ill try to provide my best explanation, i used to be a talkative and fun guy when i was young (my teacher couldn't stand me in promary school) but in my secondary school(your high school age) I remember the first few weeks for me were a struggle because i tried to make friends with others but they didn't really talk much with me.. The harder i tried the more rejection i faced. And slowly i began to develop a fear of rejection, and would rather be by myself and separate myself from others. For the entire 4 years of my secondary education period i had few friends, and many of my classmates call me a nerd because I studied too hard. And i did very well for examinations but i failed in socialising with people. I thought examinations were making me successful. And i loved to do so well to make my friends who rejected me feel inferior. But that did more damage. Slowly my friends begin to realise that i was that nice guy anymore.(i was known to be a nice guy, i'm pretty shy and unassertive) They were gossiping about me(which i hear sometimes) some said i was proud(thinking back that was the time i needed help, but i didn't get it). Slowly i began to learn the lie that friends will critisise me. And this sparked off my self-absorbtion behaviour.
I would think that the long period of "being rejected" has made me who i am today.. I dont know if puberty/genetic influences are part of the cause but I would blame it on my thinking(interpretations of events). When someone doesn't smile at me when i say high I immediately took that as a rejection. If they didnt hear me i interpreted that as I was unlikeable. I hated myself everyday, until this day in college Im struggling with my self-esteem. I can relate to your situation because my parents used to tell me to socialise more, but I hated doing it. (I'm 18 now and sometimes my parents still scold me in front of strangers, "Say hi to uncle X" and reluctantly i would say hi just to please them. And my parents also told me that if i dont talk to strangers i'm making them seem like they are not good enough for me..
Self-absorption can be caused by both self-hate and self-love, both which caused by you believing in a lie. Self-hate is about you hating yourself because you're inadequate, ugly and that nobody will like you, that you're social skills are poor. However Satan tells you that "since everybody hates you, why talk and feel concern for them?" Which is why I personally have a problem expressing my feelings to others. And there's also self-love(corrupted love) involved, that you want to avoid social activities because it's a place where you feel danger and you want to protect yourself from rejection and pain. Both of these are corrupted behaviour.
I suggest you be more open to people, dont let your self-image take control of/ influence your lifestyle. Such behaviour can leave a permanent scar and might become a that behaviour might follow you forever. Speak against lies that you're inadequate, ignore your mom's negative comments, and tell yourself that your identity is in Christ and not on this earth(your looks, behaviour) We all have an eternal identity in Jesus.
Hope i helped,
Wonderous