Something that happened to me when I was 5 really screwed me up, and it's going to be imprinted on my brain for the rest of my life. It's changed my life in so many ways, most of them being for the worse, I don't think I'll ever forget it. It's too personal to actually explain, but it's really affected me in a bad way.
Also, being a rampantly horny teenager made me throw myself into something that changed my perspective on sex and relationships. I'm still a virgin, but I came so close to having an opportunity to lose it that it made me take a step back and look at life differently. It wouldn't have meant anything if I'd done it then, it would have been pointless sex, and though I regret getting myself into that situation in the first place, I'm glad that it happened, because otherwise I'd be like the rest of the arse-holes I know who just think of girls as something to stick their dongs in.
Edit: Also, falling for my best female friend over and over again built me up and made me stronger. I was only ever disappointed, and she greatly improved my ability to get over people I had crushes on or whatever. She still feels guilty for putting me through all that hurt, but I don't resent her for it at all, I actually thank her, because if I hadn't had her to constantly beat me down, I'd still be a soft-ass ***** wanting to die every time a potential relationship didn't work out the way I wanted it to.