Breakup...so confused

Im so confused. Me and my girl have only been together 6mths. The whole time she tells me that I am very guarded and she needs me to be more vulnerable with her. But i told her that its hard for me to put my feelings out there. She is wonderful and compliments me everyday, i do not however. I show her that I love her by the things i do and i tell her, but i dont come out in detail with my feelings. She tells me i need to communicate with her more. So then i find out she is talking to someone else. Not sure if they became physical but the convo they have is very deep. I confront her and she begs me not to leave her. she says that the issue we have is communication. that she talked to her because the other was telling her all these great things that i was not. she said that i need to verbally express my attraction to her, tell her what i love about her and all those other things. but she wants me and wants to be with me. so then she starts talking future marriage and kids. she is up under me and always tells me she loves me. two weeks later she wants to hang out with friends. the first night is fine, then she goes out the following night and comes home at 4am. i find out tht she was out with someone else, i threaten breakup and she begs me not too. that she feels that she cant tell me certain things and fears that i would have gotten mad if she told me she was out with a diff friend. the next day she goes out again. she text me thruout the night professing love. the next day she doesnt come home. i catch her in a lie about where is. then she tells me we need a break. that she wants to hang out more. she swears she does not want a break up up i tell her no more lies. i cnt do it. she foes not come home for three days. when she returns i open up to her and tell her exactly how i feel about her. how much i care and how much she means to me. she tells me she wishes that i would have told her this sooner. she then tells me that this is no longer what she wants. thruout the whole relationship she has told me that i am the best gf ever. that she loves me more than she has ever loved and its scary. she even said this to me a week before all this. how does it change all of a sudden? how she chge like that...FYI..she smelled like perfume when she came home and i believe she was talking to someone over the phone about the sex they had. Confused..trying to figure out whats going on..