A divorce petition filing is better or a legal warning letter?

Married for 5.5 yes, having a son aged 2.7 yr. We never demanded any dowry. After few months of marriage my wife told she will be bringing TV and some other thongs from her side. I told it will be difficult to carry such things to Chandigarh, my place of living at that time, so instead she can purchase those in Chandigarh. My in law transferred 50k to my account. She never supported me during my economic hardship ( my full time PhD duration). She always quarreled with me. Her mother and sister are constantly suggesting hr to stay separately. Many times she went to her home without information. 1. I found her invovlef in sex chat with her ex classmate in Facebook. I have proof also. 2. After this she purchases a sim on her name but thrown when I came to knw. 3. She used bad language for me, my family members. No proof 4. Creates problem in my absence and talk bad things to my sister and mother. No proof 5. Now she doesn't take me to any place where we r invited. She goes with her mother. Her mother tells her not to go with me to invited party or places. I have proof. 6. Her attitude is not tolerable anymore. BT I waited and thought she will change. She use my child to emotional blackmail me. I want to give her a legal warning to divorce her. I think she might be forced to change. Plz advise. She also threatens me of dowry case, complaints to SP etc.