Why are women so damn difficult?

Okay, it's story time kids. I was dating this girl for three years. We lived together for 2 1/2 years. I have always been fateful to her and I was actually planning on marrying her. These past few months we went through a rough patch and she kept saying we have changed and we don't act like we use to. I never felt like this, but me being a guy tried to change for her. Well here recently she began to hide her phone from me and I called her out about it and of course she became defensive about it, as if I was being overly jealous and too much in her business. Well three weeks ago she had a girls night and went to a party with her friends but only to be brought home by her "guy friend", who she works. He brought her home at almost five in the morning! And she was surprised I was pissed about it. So, I question her about it and this leads up to her saying "Fine lets just break up". This threw me off guard because this did not cross my mind to be the solution, but me being a guy I let her have this break up because she was drunk and did not know what she was saying. The next day she had to go to work, but she doesn't come home. She text me and said she was staying with her friend because she needed some space. So of course I let her have this space she requested and I was just going to let it play out until she cleared her mind. That's where I made my mistake. That night she hung out with her "guy friend" from work, same guy who dropped her off, and ended up kissing him (her friend told me later on). Well a few days after that we were still broken up and she was going out more and hanging out with her "guy friend". So I had enough of this and wanted to know what was going on, remember we still lived together but she wasn't coming home most nights. Well I synced her iPad to her iPhone and begun to read her messages but nothing importent was being said. So I got her best friend to text her about her "guy friend" and what was going on there. Surely enough she told her everything without leaving a detail, because that's what girls do. And comes to find out that she had sex with him three times already and we were only broken up for two weeks (at this point). This took me by surprise...well kinda. I really never like that guy and I knew if he was giving the chance he would have done it. But my girlfriend, who I dated for three years, did this to me. I just don't get it. How can somebody do something like this and then still tells you they love you afterwards. So that's my story, if anybody has been or is going through the same situation, tell me about it.