She got engaged to her ex while she was still dating me!!!?

I'm going to try and keep this short. I fell in love with a girl who had a boyfriend of 4 years. She eventually left him for me. We started dating in December of 2011. As time went on, we were both in love and even talked about getting married someday. She wanted to get married before she was 23 (she's 19 now), but I told her that I wanted to get married after I had gotten my career underway, and when I was in my later 20s (I'm 20 now and in college). We had an intense physical relationship as well. She couldn't keep her hands off me and always wanted more. She even told me that she had never kissed anyone the way she kissed me. About 5 months into our relationship her ex got an a bad car wreck. He was in the hospital and she went to be there for him. After he got out, she helped take care of him because he had trouble getting around. I was mad, but she promised that there was nothing going on. Then, she told me she had to pretend to be his girl because he had mental issues and he might hurt himself on purpose. As mad as I was we continued dating. Everything was okay until April of this year, when she didn't see me as often. And when she did, it was only for like an hour unlike when she used to spend whole days with me. However, she still loved the physical stuff. A couple days ago she calls me and out of nowhere says she can't see me anymore and that she's sorry she hurt me. The next day her ex comes to my place and talks to me. He tells me that they had been dating for the past year, and that they got engaged on Feb. 24. He showed me the ring. I was devastated. I thought she was my girl but it turns out I was wrong. I was very good to her (unlike her ex who cheated on her). I sang to her, wrote poetry, and was very romantic. She told me how she loved those things about me. Why didn't she tell me anything? Why did she get engaged and not tell me? She knew he was coming to talk to me, so did she only break it off with me because she knew she was about to get caught? How could someone make promises and say they're in love and be lying the entire time? Did she get rid of me because he could give her what she wanted now (marriage) while she knew that I couldn't give her that any time soon, so she essentially took the safer bet?

-Also, he bought her a car, and bought a house and other stuff for her with the money he got from the accident. Did that play any part in it? We were together for 16 great months and it was ruined in one day. There is more to the story but that is the gist of it. Any feedback would be great.