Why douse my mom use my bf against me?

i do great in school and help around the house but it seems like whenever i am defint she says that he cant be over and i have been dateing him for over a year now and thought that they would consider him family im 17 and never told my mom to choose between me and my step dad when she took off and leaft me for a year and a half for him but now she insists on useing my boyfreind agaisnt me...i jus want to kno if i may be doing something wrong help...