wats wrong wit me?..am i crazy?

So yea i really want to know whats wrong with me...i think i have some kind of a problem...like maybe im crazy or something. Im always depressed and sad...i feel alone and i dont kno why. but i feel so stupid sayin omg im sad and dpressed but i am and i cant help it and i try to be happy and cheerful but i cant ndo it anymore..i have a hard time trying to pretend.then i get like MAD mood swings. the other day my mom came and picked me up from work and my sistr who  is 12 was in the backseat and i dont rele remember wat she was doin but she kept doin something and out o knowhere i through a water bottle and it hit her really hard and she started crying and this angered me more so it made me even more mad and then i was yellin and after i got yelled at i burst out in tear. and i think its cuz i scare myself that i react dat way. then i walso have thoughts of killing her and my mom sometimes just because they talk to me. and i see me killing them over and over and over again. ive gone crazy in the car b4 trying to get to my lil sister cuz she was bothing me wit intentions of hurting hr but i remember on time i couldnt get to her so i started to claw my neck and i had marks for a while. i also have thoughts of bitting ppls skin off. sumtimes i sit there and just bit myself not hard just cuz but then i try to bit harder and harder and i almost like it and i want to rip my skin off. i never thought dat my mood swings were bad...but then i hurt my bf not physically but emotionally cuz i get mad ovr stupid **** and i say things..hurtfully things and i hate myself for it...anyway i hurt him and he got over it ...but i didnt i promised him i wouldnt hurt him and i did...and i kno dat this sounds like such little kid..high skoo drama **** ya kno wit my bf and stuff but i cant hurt him again...i love him  so much and we have plans for the future but im so afraid dat im gonna do sumthing to ruin dat. plz help me sum1 i cry all the time now for no reason and i dont want to do anything anymore...i've stopped talkin to ppl...im a mess and i want to kno wats wrong wit me. plz help me