Why would my girlfriend own up and apologize, then tell her friends and family the opposite and blame me?

I've been through a very difficult ride with this girl for 2 years now. She has made several promises along the way and broken most of them. But, she seems very genuinely remorseful in person, tears and all, telling me she loves me and that she's sorry. However, she rarely honors her word or gives enough time and attention to our relationship. Then, over time, I become intolerable and my angst builds to the point that I feel as if I have no choice but to walk away. When I walk away is when she makes the effort. It seems that it's only then that she's in refusal to give up and is around more. I just don't get it! It became even more confusing as of late. I had just gotten so tired of all this! Keep in mind that she's broken so many promises, yet in person seems so remorseful and sincere. She tells me she loves me with all her heart with tears running down her face and pleads with me not to give up on us. Her behavior has caused me to walk away an alarming number of times, easily more than anybody ever should, yet each time I foolishly allow her to tug at my heartstrings and eventually I cave like an idiot and give her yet another chance, that is except for this last time. This last time I decided to end the insanity by going to her house to retrieve the cell phone I had purchased, because it's been mostly that phone that has been the majority tie that binds and her favorite way of tugging at my heart strings. Anyway, after I arrived at her house to pick up that phone, we ended up having an unplanned chat for about an hour. AGAIN, she seemed to be remorseful, expressing regret for her actions, apologizing, telling me she loves me with tears in her eyes, and imploring that I not give up on her and us, but for some reason I felt like she was playing me in an attempt to get the phone back. I then leaned up against her kitchen wall and was silent without a word. After 15 minutes I left because I had nothing left to say..., with said cell phone in my pocket. About 30 seconds after I left, the phone buzzed. I looked at the screen and it confirmed that she had immediately got up after I left and deleted an email account that was linked to that phone. Little did she know that it would kick back a message confirming deletion of that account to that phone. Busted!!! Still I am bewildered. I had no intention of digging into her messages. I just wanted the phone back so she would stop playing that game of tugging at my heart strings. However, since I did receive that message confirming email deletion, I became angry and decided to open that phone to check her messages. I was floored by what I found!!! Here's a girl that says she loves me with all her heart, with sincerity and conviction time and time again, yet she had broken so many promises. I asked her many times just to be honest with me and tell me if she didn't want to be with me. She always said, "I actually wish I had more time with you". Still her actions did not match her words. Then, I open the cell, check the messages, and what do I find? I find information confirming secrets, lies, along with printed conversations that she's had with friends and family members blaming me for EVERYTHING and laughing about it behind my back. Her behavior drove me away time and time again! She took responsibility for that behavior by owning up and apologizing, yet in one of those messages, just as one example, she stated to one of her friends, ("Well he broke up with me again for the 8th time! LOL!! And this morning I got 9 emails from him! LOL!! CRAZY!!"). I have never been through anything like this in my life, so it's not typical of me to end up in circumstances like this. That's how I know without a doubt that it was her behavior that caused the majority of this madness because none of my past history patterns any such damaging behavior even remotely. I should have gotten that phone back a long time ago because it led to the truth! What I don't get, is why would someone say that they love me with all their heart over and over again, pleading for me to not give up when I try to walk away, owning up to their behavior being damaging, then showing remorse and apologizing, then that same person goes behind my back to blame and bad mouth me, all the while laughing about it like it was a big joke with a bunch of "Ha-Ha's" and "LOL's"?! Anyway, I said all that just to say this: WHY WOULD SOMEONE, LET ALONE A GIRL THAT SWORE SHE LOVED ME WITH ALL HER HEART FOR 2 WHOLE YEARS, STOOP SO LOW TO DO SUCH A HORRIBLE THING.., TO SOMEONE THAT GENUINELY LOVES HER AT THAT?!?! WHAT KIND OF PERSON DOES THAT?? I've been through some ordeals in my hay day, but that is a very malicious game to be playing with anyone's heart, and that takes the cake. I am a fairly intelligent guy, but this has been incredibly confusing and I will never understa