Ask HN: What were your naivetés in your twenties? 16 years ago
Learn by fucking up. It's as simple as that. You will gain friends and loose some too, as well as girlfriends or any other person. But in the end, you will learn what's The Right Thing and your friends will be true friends and your GF/Wife/SO will be a person who respects and love you regardless of your flaws.
Being 36, father of three, married for the second time and doing pretty well for myself these days, I can of course only relate to my own experiences of life.
But I'm pretty sure that if I've asked anyone the question you're asking 15 years ago, I would have gotten some advice on how to get by in life, but I wouldn't have understood why the advice given to me was the right way. Experience gives you that understanding.
z80, Sweden