My X is stalking me and I'm really scared, I NEED HELP PLEASE!?

Please i need help! i was dating a boy last year that i thought i really liked, i heard he was dangerous from my friends but i didn't listen bc he was my best friends cousin so i knew i had nothing to worry about and of course i was wrong. he turned into a drug addict and was kicked out of 11th grade i am in 9th. now he wont stop calling,texting,iming, or sending me love poems. i have been crying over this! i don't feel safe! i feel like he is everywhere! i don't know what to do i don't wanna say back off because i cant be mean to ppl no matter how hard i try! please i need to feel safe again. help me! im only 15 and im petrified! and i know it sounds weird but i really and truly feel as if he would me if he gets me alone.. he is bigger then me and stronger then me. im only a child im not strong at all! i cant fight him.