What do you do when you hear some people don't like you?

I get hurt really bad easily because I'm a really nice person, sweet and genuine person. I thought when I'm nice to someone, they'll be nice but I learned the hard way that some people aren't. Then they are rude and ignore me, I get hurt. My best friend said, "Who cares if they don't like you? Not everyone will like you. You know that. We learned that in elementary school. Stop worrying about what they think of you. Some might thing you're annoying. So what? Would you rather be hated for being yourself of loved for being a fake?"