I was in love with him from past 4 years.I never realized he is deceiving me. Last year I caught him

I was in love with him from past 4 years.I never realized he is deceiving me. Last year I caught him having wrong chats with a girl, but he threatened me that he can't live without me. He stopped talking with her and blocked her. This time I came to know that he talks to the same girl. I checked with him but he lied to me. I called her, she told me that she calls him brother. He only used to talk such rubbish with her even though she blocks him. Now when I asked him to clarify everything. Firstly he got agree and same time he declined. He says he is not wrong. That girl is playing with her. But I have proof that they both talk and that girl sent me some screenshots where anyone can find that he is having feelings for her. I already have given him chances but he doesn't realize his mistakes.He already threatened me about suicide and everytime I melt down.Now if I forgive also, how can I trust him?I got mail he will marry me at any cost. Is marriage the solution? Kindly help me.