WHy am i always t reating my gf like this?

ok for something stupid thng like just cause she woudnt let me check something on her laptop, i got all mad, and i start to yell at her, and whip her with this earphone cord, i pinch her, i tickle her till she cries i take thingsfor her, break her stuff sometimes why am i doing this to her. her name is mandy d. shes on here, an she askes qestion too.