I would describe it differently, so I don't think I had it as bad as you, but I would say that my parents did not adequately prepare me for responding to bullying, and I likely did the exact wrong things as a response. I cried. I showed weakness. And the bullying continued for a long time - most of my elementary and secondary schooling.
I would say that I eventually formed a self image that went outside how my peers treated me, but then again, decades later, peer interactions can trigger strong emotional reactions.
Good job getting it together - that must have been a tough row to hoe.
From as far back as I can remember until I left for college my mom berated me day in and day out about how my of a piece of garbage I am, how I ruined her life, how literally everything I did was wrong, and how I'm the reason my dad is an alcoholic. Despite me being a good kid who never got into any trouble.
I was bullied in elementary school, middle school, and summer camp for being a runt (literally off the growth charts) but it didn't actually bother me, I already learned that I was garbage from my mom.
Been through years of therapy... I don't think I'll ever be able to really recover, the best I can do is move forward.
My family recipe, taught to me decades ago? Take normal discipline, punctuate it with crying and guilt of being a single parent, then sparingly frost with golden phrases like "you look just like your fucking old man," and "don't think with your dick like him", and "women who like him are whores...", and "look at how he sits" or "[laughing] look at his writing"...
Stir in some bad grades. Maybe a latent learning disability.
Let simmer alone.
Viola! You get a 12 year old pushed so low that they've spent their free time figuring out how animals are euthanized, how much to use on themselves, plotting ways to get their hands on it...