how to proceed with divorce?

I got married in the end of 2012. In starting eveything was good. But just after 2-3 months, my husband started behaving badly. He used to give all his time to his friends and going to Gym. When I asked him to give some time to me also, he said "main kya tujhe sar par lekar ghumu?". Wives have many expectations from husbands. However I stopped asking him to give me time. Now he has found a new reason of fighting "My Mom". Every time he uses abusive language for my mom. My mother only selected him and now she is the biggest enemy of his. According to him, whenever I go to mother's house, she only teaches me wrong things and the fight starts between us. But the fact is his nature , attitude & ego (he still thinks that he is living a bachelor life) are only responsible for all this. Now I am 4 months pregnant. They have now started torturing me mentally. I just can not stay in that home. My mother in law and my husband are doing all this. My husband also said this I dont care whether you live or not, give birth to this child or not. My mother in law also tortures me very badly. I have been staying with my mother for last 2 months. I want to stay with him but I know he will never change. Please suggest me what should I do. As far as I remember, all this started when I took out his affair with a girl in front of every one. Please suggest what should I do. Should I file a divorce case? Whay should I do. I am mentally very upset because of all of them.