How can i make my girlfriend of two years love me again?

i have been with my girlfriend for about two years now. at the beginning i was the only one serious about our relationship, and i caught her talking to other guys behind my back. i got a little possessive and as time went on i became more controlling. and around our one year anniversary i started to be mean. at the time i was blind to how much of an asshole i really was. i was doings things like yelling at her when i got mad, and cussing at her when i was mad. it has been a 50/50 happy/unhappy relationship for about a year. about a week ago i had something happen to me that never has before. i heard a voice in my heart, tell me to make things right. i believe that voice was god. i wrote her a letter explaining how i felt about her(i love her alot), and told her that i wanted to start over. i finally had a revolution and i wanted her to be happy again. but when i told her, she confessed to me that she was leaving. she had been planning to leave next month, because she wasnt in love with me anymore. she said she loved me, but she wasnt in love with me. she is my heart and my soul, and i want a chance to show her how much i really want to be the man for her. i want to make her happy again. how can i make her give me a chance to show her that i am a changed person?