how can i get over an impossible love?

i fell for someone i shouldn’t of fallen for. so you can know what i mean, im a 26 yr old woman, i am a lesbian, i fell for a female that i shouldn’t of fallen for. 1st off she is taken yes she’s bi but she’s taken. and i have a girlfriend already of 4 yrs. i need help getting over this person because its ruining my relationship. i cant even say i love you to my girl. to top it off i know some people may say its wrong but i dont think it is i believe in god and god loves me the way i am i know that but i fell in love with my cousin. i know its love because what i felt for my girlfriend before is what i feel for my cousin but 100 times worse. i wasnt planning on falling for my cousin it just happened. her personality the way she is the way she dresses and looks i just find she is beautiful no matter what she wears. plus her man doesnt deserve her he mistreats her and lies to her. anyways i just need help getting over her. please i need advice.