Should I dump her or give her a chance?

Should I dump her or give her a chance? A week ago my girlfriend had stayed over at my house and her phone was lighting up with messages. When i went to put the phone on silent i noticed that the messages were to her best friend in which my girlfriend's ex was the subject of conversation. Apparently he was starting to see someone else and the message that threw me off was "I miss him like crazy". Right away, as I could not hold it in, I confronted her about it. We've been dating for over 4 months now and prior to her coming over (a Monday) , we did not communicate well the entire weekend that past. Mainly due to the fact that I was extremely busy (we work completely opposite schedules) with work and super frustrated with certain things that happened at work. She mentioned that in the past few days it didn't seem as if I was happy to be with her and that she missed the mutual friends her ex and her shared because those same friends are the ones she grew up with Mind you a week before this text we were on vacation for the week in the Dominican Republic and had a great time so I don't know how 3 days of miscommunication could cause her to say she missed her ex. I wanted to call it an end right there because I was great to her. I just couldn't understand. She was absolutely devastated that I wanted to break up with her and mentioned that she has no feelings for him and that the only reason she said that was because of those two reasons. A week has past and it still bothers me even though she's trying her best to prove to me that this is what she wants and a future with me is all she desires. Its been nearly 2 weeks since reading that message and she's been trying really hard to rectify the situation. She dropped off a card and a 4 page letter attached to it at my work explaining her affection for me and how sorry she is and that she would never hurt me again and she wouldn't let us end without fighting her hardest. What do you think? Should I give her a chance or let it go?
Should I dump her or give her a chance?
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