While having sex with my girlfriend she tells me that she hurts, she told me that from the first time we had sex, and I thought that I was going a little fast when I was inside her, and I asked her if I was going a little fast and she told me that this was not the case and she didn't explain it to me clearly, so at the second time, when I was inside her, I tried to be as slow and gentle as I could, but she was still hurting.  She told me that it was her first time having this kind of situation during sex. From that time we never had sex again. I tried to have sex with her but she was refusing. She said to a friend of mine which he is also a very close friend of her that I was no longer attracting her sexually.

I try very hard to find out what is going wrong and I get more confused as I try ,I don't know what to do, I feel that I am totally responsible for this situation and I can't do nothing about it.  Why she hurts? Can this problem be overcome?

The likely explanation for this pain is that she is not sufficiently lubricated. And since a woman's lubrication is triggered by being aroused, if she is no longer attracted to you, then that would be the explanation. While this is sad, there's not much that you can do about it. If she'd had these problems at other times, then I might say that there was a possibility that she doesn't make sufficient lubrication and you could try an artificial lubricant, like KY Liquid, but from what you tell me, this is a relationship issue and unless you can change her mind, it's time you started to look for a new girl friend.