Divorce

We have been married for close to 4 yrs. We have a 2.6 yr daughter too. when I was in 5th month, i saw some sexual chat with one of his colleague and questioned my husband. He told sorry and it is a mistake and will not repeat. Even after that he keeps chatting over phone with somebody in super market and carries his phone to washroom always. My husband always chats with somebody on Whatsapp and facebook and deletes that messages. Recently i got to see some messages coming from one number which is saved as a Boy's name. But the messages were so caring and for sure not from a guy. So I called the number and it's a girl and she didn't revel the name. I asked my husband about that and he told me I can't do anything if you keep suspecting me. I told him that I can't live with him like this and he said OK for that and didn't speak anything else. I can't say about his physical relation but for sure he is cheating on me. Kindly suggest me how should I go about this. I didn't discuss this with anybody in family and DIVORCE is a very big thing in my family. But I can't bear him like this and imagining him being close someone else and living with me normally is not acceptable to me.