What should I do? I forgave her for cheating, I just feel so empty.

Me and my girlfriend have been together for awhile now, and recently she had told me she might be falling back in love with her ex. I took this in a told her it was OK and that I still loved her unconditionally. The subject was dropped. Well today she told me she was going out with her parents, and all. So I didn't talk to her much. She gets home and confesses this to me. She lied to me she went to her ex's house. They talked and began to have sex but she stopped him seconds into it, and made him take her home. She told me she regrets it, and seems genuinely sorry. She told me it meant nothing and she missed me the whole time. She really hurt me. I talked to her and asked her why she did it. Her response was, she had to know if she still cared about him. She came up with never loving him, only the feeling when she was with him. She says she loves me so much. I believe her, so I gave her another chance to show me she means it. . . I forgave her, I know she is sorry. I know she didn't mean it, I just hurt from the lies of today, I feel so betrayed. Is there anyway I can cope with this, I've talked to her all night, I've even been able to smile. I just feel so blah. Can anyone give me some advice on my situation?
What should I do? I forgave her for cheating, I just feel so empty.
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