Why does she do this to me?

Theres a girl im in love with we have known each other for three years well I left for a year for reasons I will keep secrate but when I came back she has a boyfriend she likes me back but she cant decide out of us and I tell her she rips me apart when she kisses him in front of me and she talks about him all the time and I’ve tried just leaveing and foregetting about her but it dosent work what should I do?