most of us don't really have any specific knowledge about what we could have done better, just the hindsight to see how situations turned out, and so we now know that we could have gotten a different outcome and maybe wish that we did.
For example: "If I knew then that chick was a psycho, I would not have dated her."
Or, "I had the chance to get together with a really cool girl and I didn't, and now I regret it."
As far as sex goes, it's not that complicated. I don't really know anything now that I didn't know then, except I didn't know anything about STD's. All I knew was to use a condom or else your dick will fall off.
As far as how to have a real relationship, I didn't learn anything about that until I was in my 20's. And some people will say that you can never know it all when it comes to that.