Boyfriend cheating on pretext of marriage

I had a relationship with my boyfriend for 5 years as of now after he promises to marry me we had a physical relationship only after 3 years. Recently, I caught him cheating me with another girl which their relationship was earlier than me around 8 years as of now. The girl confronted everything about their relationship status and send the screenshot about the messages the numbers of calls through whatsApp. But he denied all the allegations made by the girl, saying that he is not interested to her. According to the girl they even plan for the marriage but my he never accepted about the fact. After facing all this problem I then planning to break up and move on and he blackmail me about my personal photos which he kept in his phone make me scare for many times. Then he convinced me and promise that he won't talk to her again. He ask forgiveness and I thought he realized and forgave him, after some days I got a message from the girl again stating that your boyfriend keep on calling me please stop him. After all loosing my control I talk to his parents about the matter going on through the phone, according to his parents response they knew about the girl for a long time but they never accepted her to be their daughter inlaw. They said they knew only about me as of now and they ask me to cool down and concentrated to my studies and when times came they will talk about the marriage. With this issues I have been suffering for more than 1 weeks and I could not able to share to my parents due to the religion different. I want to see him getting punish for cheating me and blackmailing me for which I'm going throughout.