Case of Forced Resignation after Performance Improvement process

Dear Sir, I had worked with a software company for 10 years. They put me on Performance Improvement process for 2 months. The goals were too stringent. At the end of 2 months they told me that your Performance is not successful. And that's why if I don't resign before 12 o'clock same night, they will terminate me. The grounds for Performance not improving was not so serious. But I was scared. I resigned before they terminated me. They also told we will give you some money - 1.5 months salary at most. Which I have accepted. But it is highly unfair. My Performance was not so bad to deserve termination. Again I was scared that if I raise my concern, they may harm my chances for next empLoyer when Background check is done. I had sleepless nights. Finally I wrote to the HR. They are telling you have resigned by yourself. I have a Resignation letter from them. Also Full and Final Settlement done. What are our chances to take this to court ? Thank you Regards Arabinda