I have had a similar problem. I'm also a girl, now almost 20 years old and I've never thought of myself as decent looking. People do also ''confirm'' this perception of myself when they tried to bully me, telling me I'm ugly, this and that. So I started putting that phrase in my head, and everytime I would get a compliment, I would just 'throw' it away by telling myself they're just being nice.
The thing is, you'll always look at yourself like you're not the prettiest one or even like you say 'not pretty'. Even the prettiest girl in class, won't think she's that pretty. It's the way we see ourself.
I now have a boyfriend for almost over a year, and I've never imagined me having one. He's actually very popular, very likeable and has such a pretty face too. But who would've thought he would have interest in me?! Like for real? I told myself there are so many pretty girls on this planet and you chose the ugly duckling. But he made me more confident about myself and now? Now, I can already tell myself I'm really not that ugly what I always thought, it's progress already... but I'm still working on it.
You shouldn't stop taking care of yourself. I'm sure as hell you have a 10/10 personality and honestly, that's what counts the most, my dear. I've always been proud of my spontaneousness and my personality that I didn't even care what others might think of my appearance and I think that's what brought me here today. Please, if I can give you one advice, keep yourself clean. Have a morning and night routine, eat what you want but don't overdo. Work on yourself as in positivity about yourself, confidence and enjoy life. Because you are pretty.