My previous employer(Boss) made me to write an apology letter.

Dear Sir/Madam, When I was working as a fresher in IT field i.e my first company(small startup). I was assigned with a very bad & incomplete project to work, which frustrated me. During that period, I had a whatsapp fight with a female colleague(who is my boss close friend's daughter) regarding some movies. Later, she threatened to lodge a police compliant against me. But, she was the one who always keep on teasing me whenever I answered the questions asked by my boss relating to some information in office. For which my boss(whose father is IPS officer) threatened me that I will be put into jail if she files a FIR under 66A IT ACT & also he told he will remove me from the job. In order to continue myself in job, he forced me to write an apology letter on a white sheet saying that "I have used bad words against her & others" made me to sign(which I signed wrongly not my Original sign & I didn't mention whatsapp in letter), then she & boss also signed on it . On the backside of letter he wrote " this is first & final warning & if this repeated my contract will be terminated ". This incident happened during sept 2015. I left that company in mid of June 2016. When I asked to give that letter back, he denied & told that he won't give complaint to police station & told me its a crime he can't give me. Now, I am very much worried,depressed & afraid will that letter affects my future career in IT field. Kindly, redress my grievance with a necessary response.