Feeling low and jealous

Ok..so..
My whole situation...its complicated. Its hard for em to explain. Its hard to explain why i feel like this. I doubt its depression, but..im just really confused.
For the past two years, i have felt so low. Im only 13. I dont know if this is jsut growing up or...i just dont know.

Basically, at times, i just feel so so low, and jealous of my friend. I feel like nobody likes me, i'm worthless. That I'm not good enough for anyone. And I'm jealous of my friend because i feel like everybody loves her. every time i try make a new friend, they always like her more. I feel like she ruins everything. I don't know whos at fault, me or her. Like, I could have the greatest day in the world, but as soon as i log into facebook and see someone has commented on her wall and not mine, i plummet. I always compare myself to her. She tall, blonde, pretty. I just feel second best. No on likes me, I feel like I have no friends. I always wonder, what am i doing wrong? What is she doing to make everyone like her that I'm not? And why me? Why do i have to go through this?
I get so affected by this. someone help
And sometimes whenever my friends ask me out, i get a really bad feeling, and i don't want to go
I hate it. I used to have a high self esteem, liked my looks and personality, had a good amount of confidence. Now its all gone. I've started to hate her, and myself.
I'm only 13. I keep thinking over and over what makes me feel like this.
I'm just so sad and jealous of everything. Ive had enough, you know. please someone tell me if its just a phase or something. it really gets me down. It probably isnt depression, if it isnt sorry, i just dont know where else to go for advice. My mum and my sister know everything but it doesnt change anything.