Ah junior high and school dances. Those were the days. You get fucked up when you get to highschool if you have the mindset that you will be able to stay with one girl for the next four years. If you read all of this I think it will help you understand things better, really I do. Unless she is also thinking like you, it will be really hard to keep a relationship with her.
I remember my girlfriend I had in junior high. I dated her on and off for 2 years. At the start she was really sweet and didn't care about being popular or anything like that. Before me her ex didn't treat her the best and he talked to loads of other girls when he was dating her and didn't seem to care.
I actually cared about her and well, I really loved her to be honest. And tried my best to do everything I could to make her happy. But sadly, us against the world didn't last forever. I didn't do everything I wanted to with her because I was shy and I just wanted her to be happy, and didn't want to mess up with this girl I loved more than life itself. What I did do wasn't enough for her I guess. And now she's exactly like this girl you describe. At first she couldn't have cared less about what all the popular kids were doing and was just sweet and a really nice person. I'm in my senior year of highschool and she literally isn't the same person I knew in 7th and 8th grade.
Everyone in our generation is on snapchat and instagram and they are living the same superficial life in which they must be instantly gratified. Once deep and thoughtful people get sucked into conforming to these rather pitiful and extremely superficial values to fit in with our generation's society.
Our generation rests on this concept of instant gratification, only looking for what's on the surface and making themselves seem as unintelligible and superficial as possible to fit in with the society of our generation.
Think about it. Why are all of these fast, unintelligent, half-deep and superficial characters so popular with our generation. "Lil b the Based god" - couldn't present himself to seem any more unintelligent or flat - yet he is considered one of the coolest and influential figures of popular culture in our generation.
All of the popular music in our generation is about promiscuous, shallow and half-deep women that present themselves in way to seem like they couldn't have less respect for themselves, their bodies, or deep thought and real relationships. (ex. miley cirus niki minaj, iggy, etc.) What's considered "cool" to do in our generation is for girls to go to clubs and give themselves up to as many guys as they can.
This is why your girlfriend has ended up or is starting to end up like this. She is conforming to the values of our generation. If that's the path she chooses to take then you cannot change her. It is really sad to see someone you truly love and care about get lost and go down a path which you cannot follow. It's sad to see someone you love so much gone forever. She might still be there, but she is not the same. If you are not happy with her and she is not the same person that you remember the BEST ADVICE I can give you is to let her go.
I went through so many years trying to hold on to this woman that I loved so much because I could not see living my life without her. After all of the good times we had

But what I realized is that you cannot bring back the past, nor can you live in it. You can't change what has already happened or who someone is going to become. Don't try be with someone who will only go half-deep when you want to love them with all of your heart. You either need to step back to their level or find someone who is at your level.
The best advice I can give you is to live for now. Forget about who she was and forget about the good times in the past. Because that is not now, and you cannot bring it back or change who she is. It's hard to let go of someone you love so much but my advice is to look for the other girl out there who is waiting for you that understands who you are and loves you for who you are. Someone who wants to go as deep as you and be loved and share all of the moments and memories you want to have. She is out there, I promise dude.

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Looks long but only took me 5 mins to write.