How can i stop being mean?

im a very mean person i don't know really why and on the other hand i do sometimes i say stuff that i shouldn't im very loud, obnoxious, and people take it the wrong way, i feel bad sometimes after saying this stuff, like calling people fat, or ugly, or retarded i feel bad and also im a girl so its kinda not cly when i have a trucker mouth. i keep thinking im like this cause i Always got picked on when i was younger now im 15 and i think i just have my guard up so know one will hurt me, also i have a very stressfull life so i do tend to swear and take my anger out on other people.. but i want to change and i don't want to be mean anymore i wanna be nice! help!!! :)