My Girlfriend and I Need Help?

Me and my girlfriend have been together for just under a year. We love each other to death and have been through alot for each other. But now that we've been dating for so long she's started to treat her friends a lot better and me a lot worse. She went to a party the other night and promised she wouldnt drink or smoke and when i went to pick her up she told me she had lied. That night she was really apologetic but the next day she made it my fault some how and didnt even apologize. She texts her friends so excitingly and me she doesnt do that anymore. She never includes me in her plans with her friends either. She admits that she puts them before me and treats me bad and says she'll change but she never does. I love this girl to death but sometimes i feel so alone. I talk to her all the time about this stuff and sometimes its a casual conversation and sometimes it turns into an argument, which we do a lot. Shes in High School and i'm in college and we both really want to stay in this relationship forever but i have my doubts and I hate to think about it. I have trust issues because she lied to me about the drinking and smoking at the party after assuring me the whole day that i have to trust her. I want to stay with her but she's just not as caring as she used to be..and no matter what we talk about she never changes for me. She just doesn't care about my feelings that well and I bend over backwards for her. When she was sick i laid in bed with her and took care of her all day. When she gets in trouble i bring her magazines and stuff to cheer her up. and when we get in arguments i always bring her flowers and a love note apologizing but she would never do these things for me. She appreciates them and everything but she just isnt like that. The way she gets with me when we argue hurts too, she can be so incredibly nasty and just cut me really deep. Honestly i just dont know what to do. I love her so so much and i would go through anything and everything for her...I just wish it were easier.