Mental Harrasment and Blackmailing

Hello! I am 28 years old bachelor boy. I have a friend (Girl) who is 18 years old. As I live alone, she used to visit my house with food and stuff and was very caring towards me. One day we indulge in sex and during sex (before intercourse) she asked me if I will marry her and despite being very sexually aroused I said no and controlled my urges and set her free. Next day she again came to my house and again started being close to me and without any commitment we had intercourse. From that day onwards she is visiting my house and have sex with me being just simple friends. She is very practical about it and tells me that she enjoys sex with me that's why she comes to me, but twice or thrice in a month she suddenly make a big scene and force me to marry her and blackmail me that if I won't agree she will write a letter blaming me and commit suicide. She has even written a letter about our physical relations 6 months before and gave it to her elder sister and mother. Her mother called me and somehow I managed the situation. But still that girl comes to me and force me to sex (without protection) and blackmail me now and then that if I will make more female friends or marry any other girl then she will again write and letter and run away or commit suicide. I hate her but still I have to meet her and have sex with her in fear that she might spoil my image. Her mother also tells me that if her father comes to know about this physical relation then he will kill all 3 (the girl, girl's mother and me). Her mother knows about our physical relation but she still allows her to visit me as the girl is very rigid and her mother is scared of her that if she will stop her, she might tell everything to her dad and create a big scene. I don't know what to do now. I am very much scared as my parents are searching girl for my marriage but she won't let me marry at any cost. I cannot tell this to anyone because no one will believe me that a girl can force a man to have sex. People will blame me only for everything. Please help me and advise a legal procedure to come out of this trap.