My advice is to lighten up. Realize that girls are just girls, fallible f'd up human beings just like your mother or sister or teacher. Many guys put girls they consider really attractive up on pedestals, thinking there's something about them that makes them so different from any other girl they know. They're not. They may come in a nicer package. But underneath they're very much like any other girl you know, young or old.
Also, just look for girls that you enjoy being with, and don't size everyone up for marriage and forever. It's easier to say something to a someone you'd just like to enjoy a conversation with than the future mother of your children. Today is today, and you can't control the future. Maybe talking to that girl will lead to romance and maybe it won't. And acknowledge you don't have much control over that. Either it will happen or it won't. And you can't force it. So just relax, be yourself and see where it goes.
We all say dumb things sometimes. Don't be so full of pride that you can't admit when you did say something dumb. But at the same time, just because the girl isn't diving down your pants after your first conversation doesn't mean you did or said anything wrong. Love takes time. And it develops in all different ways. Sometimes you're hot and heavy in a week. And other times you're friends with someone for years before taking it to the next level. It all depends on timing, circumstances, personalities and so on.