How you carry yourself, or the level of confidence you exude, is much more important than whatever flaws you're seeing in the mirror. A guy is not going to notice that you didn't put on lip liner or you have a zit on your forehead (unless it's one of those monstrous things that looks like an alien is about to pop out of it).
Nobody scrutinizes our looks nearly as much as we do. Flaws are subjective, anyway. You might think you have a big nose, but a guy could think it's cute. Haven't you ever said, "I'm having the worst hair day," and a friend says, "Really? I was just thinking it looked nice today." She might be lying, of course, but probably not. People just don't notice what we notice, and even if they do, they don't care nearly as much as we do. They're too busy worrying about how they look to you.
There are limits, of course. If you show up for a date with unwashed, matted hair, bad breath, crust in your eyes, boogers in your nose and broccoli in your teeth, yeah, a guy's gonna notice that.
Self-confidence is much more noticeable and attractive than most physical features. No one is physically perfect, anyway, so forget that notion and roll with what you've got. If you're not feeling confident, yes, fake it. No one will know the difference, and fake confidence has a way of becoming real if you do it long enough, since it's really just positive thinking.
So yeah, sleep in some days and pull your hair back in a scrunchie. We'll still think you're hot.