OP's weird and concerning post aside, is an interesting one to think about. I guess there are loads of things I regret doing/not doing, but it's impossible to know where things would have led if I had taken a different path. Might have made me much happier, or might have made me much less happy, who can say.
If I was choosing one thing though, I think it would be to have gone to boarding school. When I was in secondary school my parents floated the idea of boarding school and asked if I'd be interested in going. I said no, I think mainly because I was a bit shy and timid, and scared of going to a new environment where I wouldn't know anyone, and I'd miss my friends. However, looking back I think it could have been a chance to really develop. Also looking back, while some of my school friendships were good and positive, I think some of them were really unhealthy in hindsight - a clean break (with the opportunity to see old friends that actually liked me in the holidays) might have been good. Would have got me away from my parents too - while they always cared for me, they argued with each other a lot, and in hindsight it was definitely not a healthy relationship. I think that tension at home probably affected me a lot more than I realised at the time.
Who knows, might also have been best buds with some lord's son and get to go and holiday in his spare castle or something.