You focus far too much on what you think they're thinking. Maybe you're just a bad reader of people.
I doubt I've misjudged what an entire sex thinks about me.
It'd help if you don't constantly look at every girl as a potential relationship, as I think you've really built up and idealized this whole sort of love at first sight type reaction that doesn't really exist.
Just talk to them and don't think about relationships and don't constantly scan them for body language trying to figure out if they're interested in your romantically.
It's not even scanning really, if someone's more interested in something else, or talking to someone else, then it's clear that they don't want to talk with you or are interested in you. Same thing with quick answers in person, or in text. Same old, same old.