She cheated,what do I do?

[Chase]

Well,me and my girlfriend have been dating for 2 years.and some months.for the first two years,we were amazing.then lately I have noticed that she was evasive,and ignored me.we rarely hung out and when we did,she was always bothered.also,i met this girl while I was in the army,and when I got out,i moved to her state.so anyway,Christmas time comes and I go spend it with her.she was mad the whole day and then I finally convinced her to watch "Doctor Who" with me.I went to grab her laptop and noticed she left her email open.then I noticed her last email was sent to a guys she's been talking to a whole lot.she keeps sending her pictures of her day,her pets and random stuff she used to send me.now,she was telling him that he went out with this 40 year old guy,and she was confused about her feelings for him because he was married and with kids.he's also a friend of her dad's.she told him she had always had a crush on him and that she wanted to hang out with him.I showed her the email and walked off to cool down.I came back and asked her how far her relationship was with him.she said all they did was hang out.so then,I went into her Facebook,and read the messages between her and her best friend(they tell each other everything).she said that "He makes me so horny",and that they had sexted.so after all that I walked out her house and went to mine.she then came over under the pretense to return everything I've given her and while there,she begged me and cried and I ended up giving in and taking her back.I am now 2nd guessing this decision,I feel as if I'll never trust her.she lied to my face.she said the relationship was just a hang out,but yet,she sexted him.so,what do you think I should do? she said she didn't have sex with him,but if she lied about everything else,how can I be sure about this?and she told me that if she had not been found,she would have never told me,so what guarantees me that this is the first time she's done that?