I mean, this is definitely a case of "do as I say, not as I do", but I feel like you shouldn't be so hard on yourself for that.
That's a long chain of events and a ton of what-ifs. You made a bad decision, and you can kick yourself for that, but you dealt with the aftermath the best way you could with the information you had. Surely, you did not know how hard that job would be on your body, right? Same thing goes for the accident. Did you have any inkling it might happen? If yes, moreso than anybody else who takes the risk to longboard?
Finally, if somebody else told you that happened to them, would you blame them? A lot of smart, responsible adults goofed off in high school. My older brother was a huge partier in HS and never got his degree. He now owns his own business and is great at what he does. I won't feed you the claptrap that everyone can succeed or that could be you, just that a good person could have made the mistajmi you did.