If you realize that you didn't ever do anything mean to this girl, I have an easy solution to why she's being mean to you.
She's jealous of you.
There's always going to be people who are jealous of you. Maybe they think you're more pretty than them, you're more confident, you have more stuff, more subscribers/followers/friends on YouTube/Facebook/Twitter etc; there are countless reasons.
You just have to feel empathy for this girl. Don't pay any attention to her! She's the one dragging you down. If what she's doing ever gets really out of control though, make sure to tell a trusted adult (mom, dad, aunt, grandma, teacher, etc etc etc)! I know that doesn't always seem like the best thing to do, but it really helps if you can just tell someone. Trust me! I know from experience.
I have a couple friends who are really competitive with YouTube. I've been on YouTube WAY longer than they have, and I always find them talking bad about my videos, or just me in general. It used to make me really mad, but now I just get kind of irritated before letting it go. I don't really care what they think. They're just jealous!
First of all, though, you do really have to think back and see if you hurt this friend's feelings. Maybe that's why she acts so nasty. If you really did do something mean or rude to hurt your friend, brushing it off and acting like you didn't care might hurt her more! So be sure to apologize.
Treat others as you wish to be treated!!
I hope this helps :)