Why am I so self conscious?!

how to begin?
it started when i was about 12, when i realized that my weight was growing, when i began to notice all these flaws about me that had never bothered me before.
i began to think that my friends were excluding me, while i was the one pushing myself away.
i asked myself if anyone had noticed that i was the only (i hate this word, so i'll say: non-thin) girl in my group of friends.
there was a point in which all my friends had boyfriends except me
and i thought, "these bi***es only want to make me jealous" which, frankly, it worked.
how could i say that about the girls who have been my friends for over 5 years??
i hate being like this. i hate the feeling that i get when i see a skinny girl who looks good in skinny jeans, while i look like a ball of putty. worst of all, my mum says that i eat because of nerves, and im starting to think it is true.
another thing is that i NEVER speak in front of my friends, im always the quiet one that makes awkward silences worse. i dont know why, but i never have anything to say. and if i do, i whisper it, and if i try to repeat myself, i feel like a dork.
you know, i wasnt always like this. i used to be... happier, i guess you could say, i didnt keep to myself, i didnt care what other people thought about me, i was actually confident. and then... i turned into this.
im sorry for going all emo-ish here, but i just cant take it anymore. i feel like poo.
is this some sort of psychological disease?
or, how can i improve who i am? im not seeking perfection, just a more confident attitude.
thanks for taking your time reading this :) :/