But my past is relevant to explain my present troubles. It's not that easy. It's not just a decision you fan make and suddenly start doing. "Oh, I'll start being more social!" That's not how it works. It's a struggle to adapt. Because of how awkward and shy I was brought up, it remains a problem. I want to change but just deciding all of a sudden and saying "fuck it" isn't what happens ,as much as I wish it was.
I can relate to your situation pretty well, I think; I was an awkward kid in primary school, my interests were academic extracurriculars, playing netball until I got injured, scouts, and reading SFF alone. In high school I was homeschooled for awhile and ultimately did not complete HS. I don't know how old you are, I'm at university (I posted earlier in this thread, idk if you read it) and I'm still struggling to adapt (especially because I'm now hard of hearing, which makes social situations very difficult). I agree with you, all of these things are very very very very very very relevant, but they're only more impetus to try and change things, because you're clearly unhappy with your current situation and it seems like these were uncomfortable experiences with you in regards to how you relate to others.
I can tell you in full sincerity that, as daunting it is to begin trying, especially if you have bad experiences that cause you to behave self-consciously (and this is truly not easy to ignore, and it's important to be self-aware! finding that balance is tricky and I agree that 'just do it' can be glib advice), the only thing you can do is try. You recognise that there is no magic bullet for this situation. You want to get more confident. So then the logical conclusion is you must start somewhere to achieve your goal. And no, the change isn't overnight or dramatic. Self-presentation is one place to start but, honestly, the most important thing for overcoming social anxiety is expanding your comfort zone. By this I mean small interactions that build up over time. If something bad happens, because life never goes in a perfectly smooth trajectory, you retreat a little, reevaluate, then keep trying with other people, getting a little further as you build up confidence and self-assurance; this is how you build up resilience. Positive experiences, consciously internalised, offset negative experiences with time, space, and personal growth. Get to know people gradually, especially if you ever have an opportunity to interact with people in a new environment or situation (new grade/class/hobby). Speak up a little so people realise you're there and start to notice things about you other than you're very withdrawn. I don't know what sports you play but that seems like an environment with ample opportunity for friendliness, non-solitary hobbies tend to be.
You yourself say you're not of an age where you're actively seeking romantic intimacy, so I'd honestly advise you to try to get into the mindset of learning how to befriend girls and act normally around them. I say this with experience that if someone doesn't think of you as a potential friend and only views you as a prospective romantic interest, or primarily conceptualises of you as the 'opposite sex' or somehow fundamentally mystical by virtue of gender, it comes across as really obvious and makes it hard to have a proper friendship. It can also be intensely overbearing. I realise that there's a tendency to segregate among genders in school and HS sports are gender-segregated mostly, but at least at schools I attended it was common for all genders to mingle, increasing in likelihood with confidence and popularity (I was part of an all-femme clique in one school that was founded on a mutual interest in, you might be interested to know, anime! The girls were goths and I'd already had and dropped a goth phase, but I was accepted because I was as much of a pariah as them and equally willing to eat pocky as a social statement).
The other issue is self-conceptualisation. And I'm not gonna lie to you and tell you that people will automatically treat you better because you think well of yourself. I had decent self-esteem before I got bullied. But working on your feelings of self-worth, value, and figuring out what you have to offer the world and what makes you a special person will both be positive for your psychological wellbeing and for your ability to confidently interact with others. And confidence is something people pick up on, so, yes, it is hard to get started. Many things are hard to get started, and this is particularly hard and punishing, and it's a long trajectory... but it's worth it. Keep in mind people won't always be the kinds of people they are in high school, but there's no reason you can't start working on your personal growth now.
Be open to talking to other shy kids that you might not notice, too... it can go awkwardly if neither of you know what to say (expect silences and try not to openly fret too badly, obviously I say try because it's just a thing you have to practice!) but if you have a mutual interest or can figure out how to find something interesting in the stuff they like to talk about then odds are they're in the same boat they are in. (A lot of that type of conversation I have found is being able to talk to people about interests you don't necessarily share or invest much time in, many things can be interesting if presented the right way and to the right depth by the right person.) If you can figure out what you both like, try and ask them to hang out sometime, etc.
And pay attention to your surroundings :) You can learn a lot about people and social cues (something that's hard to pick up if you haven't had a lot of contact with kids your age to naturally develop it, you have to sort of learn it as you go along inferentially and sometimes by asking) that way, and individuals. The less you think of others of threats and the more able you feel to pick up on things like mood and flow without projecting too hard, the more relaxed and less anxious you will be.
It's honestly a long process and I wish you good luck. And if you can see your school counsellor discreetly I would suggest that maybe? They often see students who are bullied or having trouble adapting socially and can give you more specialised advice. And talking through the difficulties you're having as you go along can be very helpful for thinking your way through things and processing things emotionally, and talking about how the past is affecting your present can help you move beyond it while still being aware of what you have to adjust for.
And to be honest, while collectively your hobbies do sound like the profile of a nerd... there are many, many kinds of nerds. Interests can be played off, interests can be played up as cool, interests can be extended to relate to other people. A lot of the thing with awkward interests is how you're perceived in relation to your interests (if you're That Kid that can only talk about one thing and doesn't show interest in anyone else's hobbies and seems to be impeded by your hobby, it comes off badly, but there's no reason a decent interest in video games should be shameful). Lots of people like video games these days, it's something you can actually relate to other people with. Anime is a less common hobby and comes with connotations but those connotations exist because many anime fans in the West do. Sport is a hugely relatable interest.
tl;dr: yes, you need to take risks, but not all risks are equal and it's something you can do gradually