Why would my girlfriend of 2.5 years leave me for another guy?

People go thru a lot of changes at your age between high school and college and many want to explore new possibilities. That is what happened. She changed but you didnt. What she did by using you was selfish of her and even kept you from using that time to move on. I just hope you realize she has turned into a very self centered woman who uses people. Do not play the fool and allow her to use you again. How can people be so cruel? It happens a lot because of the selfish nature people have. And yes she might try to use you again because you could be considered a comfortable safe place for her when her other relationship fizzles. But if she did this to you twice you can bet it will happen again.