Need advice with my girlfriend

I have been dating my current girlfriend for 8 months. We have had a very good relationship and have each worked through a lot and made sacrifices to make this work. With all relationships, the foundation of our relationship has been trust and being faithful to one another. Last Monday, August 16, she did something that I never expected of her and never saw it coming.

We have been telling each other we love each other for months now. It all felt really sudden but I really do love her and I know she really does love me.

I was out of town for the weekend visiting family. That Monday morning, we got into a little fight over text messages. She ended up taking things I said the wrong way and I wasn't able to clarify by calling her because I was in the car with my uncle and mother. She told me she wanted time alone and I respected that and told her just to contact me when she was ready. We went all day without talking, which is very odd for us, and finally late that night I sent her a text asking what I did that was so wrong that she wanted to go all day without talking. She just told me she wasn't ready to talk. I insisted and called her.

In an attempt to make a long story short, she ended up going to another guy's house by herself. It was a guy she had just met the Saturday before. She went over there to watch a movie. One thing lead to another and she kissed him. She told me she felt horrible and wanted to take it all back. But what gets me is that instead of leaving, the two of them got into their swimming suits and went swimming in the guys pool. Even though she claims she felt so bad, and so awful, and could only think about me AFTER the kiss, she still did that knowing how I would feel.

In the following days, I have since talked to her and have made her tell me everything that happened that day. She never brought me up once to this guy. She flirted and lead him on and made him think she wanted more. She laid her head in his lap during the movie, and she initiated the kiss. She told me she never once thought of me until after she kissed him, and then when she did, she still decided to swim with him. She promises me nothing else happened. That he tried to kiss her more and she pushed him away. She tells me she felt nothing when she kissed him. That it was all just one large selfish act and she doesn't know why she even went to his house.

My girlfriends Mother passed away a few years back. She is only 21, I am 23. She promised me from the beginning of our relationship, that she promised, on her mothers ashes, that she would never cheat on me. And now she has. And the way it happened hurts more then I can ever express. And from day one, her and I had a mutual understanding that if either of us cheated on the other person, there would be no second chances.

Now that it has happened, I do still want to be with her. I love her and care about her more then anything. And she has been extremely apologetic, and remorseful, and tells me multiple times a day she is sorry and loves me and wants to take it all back. She says she doesn't know why it happened and never will and she will never forgive herself for what she did. She promises she will do anything to make us work again, and to gain my trust back.

I don't know if that trust will ever come back, if the respect I had for her will ever come back, if the hurt I feel when I see her will ever go away. I hate being single and I love my girlfriend so much.

Any advice as to what I should do or how I should handle this situation will be helpful.

Thank you