I’m very sorry for the pain this is causing you. I understand why you are confused. You are getting a “go but stay” message from your girl — go away for now but maybe come back later. Living in limbo like that is very, very hard.
It is sad but true that lives often diverge when people go off and have different experiences with different people. Your girlfriend is telling the truth when she says she is confused. She had strong feelings for you. Then she got involved in a life that called those feelings into question. She didn’t seek that out. She didn’t want it to happen. But being with other young people who were sharing her new experiences opened up other possibilities. My guess is that she doesn’t want to hurt you so she holds out the possibility that you’ll get together again. Maybe so. More likely, one or the other of you will find someone who has things more in common with the people you are becoming. It’s too bad you can’t take a step back and be friends since friendship was where this all started. But staying friends with a former lover is too hard for most people. They keep hoping the friendship will again flower into something more, and then are disappointed yet again when it doesn’t.
I think you should take your girlfriend at her word. She has growing up to do, and she doesn’t want to do it in the context of a relationship. She is urging you to do the same. You can’t really talk someone into a relationship although you can make someone feel guilty enough that they stay a little longer than is healthy. Give her space and take some for yourself, too. Maybe you will find each other again. Maybe not. But the growing will be worth it in any case.
I wish you well.