How can one person be such a failure in relationships?

Why is it that a person no matter how hard he tries can screw up so many times? If you truly love another why do you drive them away? I know what I have to what I need to do, but yet I still fail. Am I just destined to drive all those that love me away? One failed engagement, one failed marraige, numerous failed relationships and now the one person that I trully can see a future with I am driving away. What could I possibly be doing wrong? I am not abusive I try to be attentive and giving. I thought she loved me, she has said she loves me but today she doesnt. She is not a bad person. It is magical to watch her with her son. So it has to be me. How can I change?How can I change? How can I change? HOW CAN I CHANGE!!!!!!!!