I would have a lot of words for my younger self, and though they might not be common words of wisdom to everyone they'd be pretty life-changing to a younger me:
First, not all the doctors are right. Especially military doctors. And the medications they put you on can be ten times as hellish as the condition they claim to treat. I say this because I've had three different types of epilepsy throughout my life, and the medications some of the military doctors tricked me into getting on a long time ago are horrible things to be on. It's a complicated mess, and I'd be happy to talk about it at length if someone wants to chuck me a VM or smth so I don't derail this, but the TLDR of it is that I'll never be able to get off those meds, and trying to do so is... unadvisable at best.
Second, stop wasting other people's time and stop letting them waste yours. I know that's a bit harsh, but I used to be a lot more long-winded than I am now (which is saying something considering the length of this post). And I used to be like that irl, over actual voice. It was really, really bad, and most times it was just genuine interest in things that caused me to want to talk like crazy. I could (and probably still can) give an impromptu presentation on how math is actually useful in everyday life if you know how to apply it properly, or any other manner of really inconsequential things. I often wasted others' time in making those explanations, and in doing so I wasted my own time because all it did was take away from another person's day. All that time could've been spent doing something fun or productive, and I'll never get that likely hundreds of hours of my life back.
Third, stop being afraid of others' perception of you. I'm not the most self-confident type, even now, to the point I've had a smogon account for over a year and this is my second post. I used to want to be... normal, able to socialize like everyone else. And to a point I still do, but I've come to realize that while a social life is important there are plenty of other things I can do alone I find fulfilling. Case and point, spreadsheets. Numbers won't curse you out like a human will, won't bite back, won't judge you. And those skills can be valuable, and lead to new friends which helps with the social life (like through being able to arrange usage stats and data for a tournament). And all the time you spend worrying about others' perceptions of you is time you could have spent making those perceptions better, as opposed to being so afraid you refuse to approach others.
In other words, if I met my younger self, I'd sit him down, hand him a few sodas, and tell him "Make yourself at home, you're gonna be here a while"
