My girlfriend is refusing to get marry me because of her parents

Sir/madum. I am from karnataka MYSORE. Me and my girlfriend both are engaged in love since 6years. And we had a good matured love in the mentioned period. Even we got engaged in physical relationship since 4 years for lot many of times. in hotels and paid resorts i do have those bill document proofs for visiting.and i am having the self taken casual meeting photos and few PRIVATE PHOTOS of her which she have sent voluntarily and also private voice records from her voice in some exitmenta. And i have her hand written love letters which she promiced to get marry. and also i do have whatsapp chat history of one year. But now she is going to engage with another guy for parents force. I am not ready to lose her i cant forget those days we spent i need her to my life. Its mentaly harrasing me im getting so weak thinking about her.Its 3months without talking to her. please i request the advocates to guide me. What exact i can do by leagal means. And how to get marry her aginst her parents.