My Girl friends --- Boy friend

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I have a cute lovely girl friend, I love her verymuch and like to marry her, we loved eachother for 3 yrs, But then came those bad days. For atleast a year I left to a far place to continue my studies and missed her. I can contact her thru phone only once in a month, After 1 year when I returned back I heard from my friends that my girl friend has a new boy friend. First I didn't beleive my friends, I saw her many times freaking out with him. I asked her straight away abt this, She doesn't accept her mistake and says, she dont love me. Iam totally confused, What should I do??? Still I cant forget her. b'cos I love her verymuch, but its impossible to marry her on seeing this ???? What to do?????