What to make of this girl?

I'm 20 and sh'es 19 and we go to the same college. After her boyfriend of 3 years cheated on her, she broke up with him and I was there to be a shoulder to cry on. Of course, I didn't want to be her rebound so I waited about 3 months before telling her that I liked her and she told me she liked me too (this happened freshmen year of college). Over the summer our relationship deepened as I went to visit her house about twice a week and we exchanged our "i love you's" then. When sophomore year started, things began to change. After one small argument, she told me that we should take a break and that she wanted to enjoy the college life a little bit and that she wasn't quite ready for a relationship yet. I understood what she meant but being the sucker for my first love, I kept hope that I could have again some day. One day, she was getting to get ready for a competition in San Francisco so I made her a good luck card. Unfortunately she didn't look at it in a positive way and found it weird and creepy. As I apologized to her for the card and asking for a second chance (stupid me right?) she kept laughing it off and telling me it "was chill". Basically she said she had no more interest anymore. A week later after this incident, I found out she had gotten drunk at a party and made out with another guy and that she had begun dating one of her older friends who I had always had my doubts about when I was with her. Since she was my first girlfriend, it has taken a bit of time to get over her. I'm a good guy who doesn't drink, smoke, or do drugs. At first I was confused about how this could happen to me until I got to a point where I stopped caring. I'm moving on but I just want to know what you guys have to say about this girl? Immature? Maybe i can do better? Your advice and suggestions are greatly appreciated.