She changed in a week, and walked out of my life.

[I don't mind]

I'm new to this, figured it was a good way to find some answers. I'm 20, my ex girlfriend is 19. A little background, I was with a girl prior to this one, we were together for 4 years through most of highschool and a semester of college. We were so young, and she loved me so much, but I never really felt that true love and connection with her, so I backed off until she ended it in college, totally shattering her heart. I truly wished I hadn't of done it that way, but I'll get back to that. So I move on to this new girl, met her at work at OU. We connected instantly, ended up dating, and within a few months I felt love like I never have in my entire life. She felt it too and it was her first as well. So the relationship goes like a dream, not one tiny problem for an entire year, I was so incredibly happy, and I treated this girl so incredibly well, did everything for her if she needed me to. Anyway, after summer things start getting a little shaky, her best friend moved in with her (her best friend who is obese, smokes, does drugs, takes pride on her cheating, and controlled my ex), and wow it put some stress on us. At the time I was living about 2 hours away, so we decided to move in together to take some stress off and connect, her best friend who now hates me for no apparent reason was there too. Things go stellar for about a month, then I notice her changing a little, being less interested in me, and one week she just totally becomes a different person, to the point where she spends no time at all with me. Long story short, I can't take it, I leave back home after telling her the problems and seeing how unwilling she is to fix it. We break up, I tell her I love her, she says she loves me and she messed up, two days later I text her, she says there's someone else, and ends up getting with a guy she knew at work. So she distanced herself, found a new guy, and is now dating him within two weeks of being in a bed with me telling me I was her everything... I guess I concluded that this guy must have been growing on her for a while, I was sooo heartbroken, I haven't talked to her since, and that was about a month ago. I apologized to my prior ex for ending the relationship like such a child, saying I realized how horrible it is to do that to someone, were good friends now, both matured, but I don't want to be with her. Anyway, if you guys could just give your opinions and answers to how my ex could just drop me and move on so quickly? I mean she seems perfectly fine. The best year of my life just down the drain in less than a month. How could she do that and just be okay? Where did I go wrong? There was not one sign of this coming, and I trusted her with my life. I'm past crying and moping around all day, but it still lingers on my mind, I just don't understand, and I'm looking for closure.