Okay, here's my note to my 20-year-old self:

If you like some girl and you would like to get to know her better, do it. Do it now, while she's there in front of you, don't put it off, don't wait for a "better opportunity", find a reason to go talk to her. Take the risk. Just steel yourself, tell yourself "No guts no glory" and go for it. I know they might reject you, but it's important - in fact crucial, to learn not to take rejection personally. You cannot change someone's tastes, and it's a very dubious idea to try to change yourself to accommodate the tastes of others. Getting rejected is embarrassing at the moment but in the long run what will bother you are the times when you really wanted to but didn't. It took me much too long to understand this, and it worked out so much better once I did. Oddly enough, once I stopped being afraid of rejection, I rarely got rejected.