I'm single by choice, don't know anything about romantic relationships, have never been in one, but I've seen plenty. My advice: As far as the ones that start in middle school, not one relationship will last past 8th grade. My two friends, lets call them A and B, are dating, but A keeps threatening B with things when she won't eat or is sad or something, like "stop doing X or I'll kill myself" and they're guaranteed to fall apart quick. And as far as I know, A is also mad at B because B is "cheating." AKA has a friend, but I don't know a lot there.
TL;DR — Yes, girls are that cruel. And it has nothing to do with how you look. It's a fact of life.