Im always sad and down all the time. am i sufferin from depression? and wat do i do if i am>?

im on the brink of 14 and im always feelin sad and down/. i felt this way for 3 years now and its not goin away. idont know how 2 xplain it but i always feel upset and empty. just empty. i think i also mite hav an eating disorder. i go throu bursts of eatin heaps then starvin myself. and i always feel so fat even tho ppl ure me of bein thin and healthy. what shuld i do about this. im 2 scared tlkn 2 sum1 bout it . but i want 2 deal wif it b4 its 2 late, plz help me.