I don't think she was being mean, I mean what did you want her to do, lie? I feel for you man and I wish it would have worked out... but depending on what age you are (especially in teens - young advlt) not every relationship can be really serious. I'm 18 right now and I just go into relationships for fun, but I let the person know at the beginning that I'm not looking for something serious, but just for fun, although I'm willing to see where it goes. She's just at a different stage of life then you, and its better to know now, as opposed to a few years in.
Yeah I'm only 17 and I know the person I date now is not going to be the person I marry, but it was my first girlfriend so it was kind of a test run
now I know my boundries and will do better on the next one (hopefully). I didnt cheat or lie to her, but I was definately clingy which is weird for a guy but thats just how I am, but I knew going in to the relationship that she didnt think it was super serious considering she had 5 others before me, but as it progressed she acted like it was serious and then when I started acting like it too she got really distant all of a sudden and then it went downhill until it ended. Still friends with her though and she kind of hinted that she wanted to date again but I ignored it because I dont want to be with her anymore, when she got distant it was like she never knew me and it was the worst thing I've had to go through as it made me feel worthless (my winter depression that was raging at the time didnt help either).