Why is there no such thing as a monagamous relationship anymore?

I had this girl I really like right. We talk for bout 5 months and it was great. The was great. We had the best times together, but she couldn't be with me. I wanted to make sure it was ok for us both to see other people so I asked what kind of relationship we had. She told me as soon as I walk out her door, she looks for someone else. Another girl to make her happy. She is addict. I'm thinkin what a crock. If u don't like me just say it. She swears that's not what it is. We don't talk much anymore. I miss her a lot but I got so hurt once I seen that she was serious that I don't even think we could be friends after that. why do people feel like they can say whatever and u shouldn't care or be upset because they bent the truth in the beginning? I'm lost cause I really loved that girl. Now I just wanna do the same thing. Bounce around with several different girls and not care she does it. I dunno someone help please