Getting forced to involve in relationship

I am a married person and having a 4 year daughter and working in private sector. I am getting blackmailed and forced to be in relationship with a girl. Few months back a girl came to my contact through friend circle started a chatting on what's app. She was aware that I am married and my family background. She presented that she is married and living away from her husband just because of job purpose. She is happy with her husband and living with his family happily. Initially she was asking about the general things after three four days she has asked to go for out for photography shoot and convinced me to be with her. I agreed too go with her to learn the photography. She has done hotel booking and other arrangements from her side only. We went at beach resort she has booked a common room, in evening on beach she get closer to me and started flirting with me. unknowingly I also responded her. at night we indulge in intercourse before the intercourse we were chatting on general topics at that time she has expressed that she want to live me after twenty years after retirement. After spending one night we came back to our home. She has started messages and always asked me to come at her flat or she started to come to my office gate to pick me up. And always forced me to be with her if I tried to stopped her she start to blackmail me emotionally. There was no promises of marriage or living together at all. She was seriously looking her life with other person and was want to live with me after 20 years later. in the time of one and half months we had sex more than 10 times after that she has shifted to other city for job purpose and joined as company secretory in private company. After two weeks she came to meet me and first time shared that she is divorcee and she had gone through serious depression for two years back and now she is trying to marry with her ex boyfriend but her ex boyfriend is in process to get divorce from his wife but he did not expressed her willingness to marry with her. She done emotional drama and showed willingness and forced to have sex with her. most of the time she has taken initiative or expressed the willingness of sex with me. After this shocking story I found that she has a strong background of some abnormal behaviour and I clearly told her that I am not able to carry forward this relation and would not be able to fulfil your wish in future. I started to keep myself away from her. She started to call me and forced to give clarification about my behaviour. she offered that she would cancel her plan to marry with other and she does not have any issue about my family commitment just want love and caring from me. At present she is forcing me to be loving and caring with her. I tried to avoid her calls or respond to her on what's app. she says that she will spoil my social image and she will take her revenge as well as saying that she could not be able to perform her job just because of me. I tried to understand her expectation but she just says that she want to behave like earlier which is not possible for me. She has started to use slang words and started threatening me through what's app as well as she calls me anytime and gives deadline to start behaving as per her expectation. She message me and threaten me about legal action. She blackmailing me like anything and forcing me to love with her and start relations like earlier. I am very frightened about her behaviour and anxious about my job as she has tell me that I will file case and take away my job. She text me that she wants to screw my life and spoil at every sense. If I do not started to love her love her she will come to my home and meet my family to live with me. She always talk very rudely and gives deadlines to convey my stand. I have already conveyed that I am not able to go forward in this relationship. we both are living in different cities at present. Experts, please help me to handle this situation. I am very much frightened and lost my peace of life due to this. Please... please... please.. help me.