Let me know,
Your question is like you are going to the Doctor and saying ” I am feeling tired” . What is the cure?
You have not told us anything about you and your chachi. How old are you? How old is your chachi? Is your chachi a widow? Why do you want to stop this relationship? Are you getting married? Do you live with your chachi? Where are your parents? Etc etc.
Unless you tell us all the facts, how are we supposed to give you advice?
Has your chachi tried to blackmail you? What do you mean that she refused to end the relationship? What will she do if you ended the relationship?
Give us more information as comments and we may be able to give you better advice.