I wouldn't be beating myself up over this if it weren't my fault, but to a certain degree, I think it was my fault. I feel like I should've made more attempts to socialize with other teens
I think this is an illusion created by hindsight. Everyone feels that they should have done more, and everyone feels partly to blame for their problems, but at every point in your past you were faced with a choice, and at each point you weighed the pros and cons, and at each point you decided (for whatever reason) that making those attempts wouldn't be worth it, or that it would only increase the amount of pain you were experiencing. At the time, you were always acting in a way designed to minimize the amount of pain you expected to experience. Because that's what the brain does: it protects you from pain. And often it uses a milder form of pain to protect you from what it predicts will be a bigger form of pain. You experience anxiety at the thought of socializing with others because that smaller pain (anxiety) protects you from a potential bigger pain (being publicly humiliated or rejected). So, the whole time you were "wasting" your teen years your brain was doing its best to minimize the amount of pain you experienced. Your brain, iow, was doing its job.
The problem here is not with your actions. The problem is with your expectations. And those expectations were almost certainly created when you were a very young child. Before you could even speak you probably came to expect pain from reaching out to others. And this prediction led to a pattern of avoidance. Those predictions probably had something to do with your autism and how other people related to you. There is no possible way that you, as a young child, could have had any responsibility for the formation of this prediction and its adaptive response. So there is very little you could have actually done about this. Even knowing that isolating yourself was bad, and feeling that you should be doing something to change it, will not, on its own, change the kinds of predictions your brain is making or the brain's reflex action of avoiding pain. Mental illnesses are like physical illnesses; we do not expect people to cure their own diabetes, and we should not expect people to cure their own anxiety. We have science, doctors, hospitals, parents, school counselors, and therapists for a reason.
Saying that you are not responsible for being where you are now does not mean that you are powerless or that you are not stuck with fixing the problem. You will have to do your best to address this issue, and you will have to do most of the work on your own. But blaming yourself for having a brain that avoids pain, for having experiences that set those expectations, and for avoiding situations that you predicted would create more pain than pleasure (which is completely rational, btw) is not going to help. The regret is something you are going to have to live with, but it's not too late to make changes, and your life is not over by any means. The brain is a powerful organ, and if it can find a better way to deal with your social anxiety or make better predictions it will. Most likely, you will need some kind of help for that. Like
, I recommend self-compassion (CFT) and CBT.