I know right? I mean they actually made a big deal when Taylor and Calvin Harris actually celebrated a one-year anniversary, because that was a milestone for any relationship of hers.
As for why it never lasts, I've read different things from different guys she's been with - I think you may be right about insecurities, because many of them say they dumped her because she was smothering them with her clinginess and neediness, while others complained she wouldn't have sex with them. But as much of a narcissist as she is, I wouldn't be a bit surprised if she actually is a jerk and that has something to do with it as well.
Still, just the fact that there are people out there who always praise her everytime she hooks up with a new guy, because it somehow shows she's "so smart" because she "doesn't settle" is mind-boggling.