Atelophobia

The fear of being imperfect.
The fear of not doing something right.
The fear of never being good enough.

A mental illness. Due to people striving to reach perfection. A "perfection" that simply does not exist.

You blame yourself when he doesn't like you back. You blame yourself that he likes your best friend more than you.

Ugly. Boring. Plain. Unwanted.

Why would you feel all this? Just because some guy doesn't know you exist?
No--wait. He's not just "some guy", he's the guy. And it's frustrating that you're not his girl.

Multiple attempts to get him to notice you.
You found out his favourite book? You immediately read it.
He loves sushi? Well, you've hated seafood since you were 5, but you'll try it again, just for him.

Reading those love quotes that seem to perfectly describe your feelings:

"I think of you in colours that don't exist."
"You don't understand, there's isn't one person in this world that I want more than I want you."

How could someone you're giving this much attention to, not like you back?
Well, he doesn't. And you have to accept that.
You can't change it, nor should you try to.

Don't change yourself for anyone (unless you're like crazy or have a unibrow, gotta deal with that bro).