Why does my girlfriend treat me like this?

I apologize for the long post, but I really really need some advice about my current situation. I've been with my girlfriend now for a little over 6 months and we both love each other and started saying it about 3 months into our relationship. I'm 23 and she is 21. I know were not going to break up or anything and I'm not planning on leaving her because I care greatly for her and I'm pretty positive she might be the one. She has told me the same thing numerous times. I know it's a little early to say that, but we both truly feel the same way about each other and get along great! This is where my only concern is....she tells me all the time how much she loves me, how I'm the one, how well I treat her, how happy I make her, how much she loves our sex and when we make love, and she tells me that I'm her everything. Sometimes though she treats me like crap and at times she knows this but is too stubborn to apologize. I have never once cancelled plans with her except for one time when I was sick and had some sort of stomach virus. She cancels on me all the time and always blames it on school and how she is tired and stressed from it. I know she is telling me the truth and stuff, because she is taking 6 classes this semester one of them being an online class. I have given her space and allowed her to have so much time to do her homework. This was going great for awhile, but had its fair share of ups and downs. When we hang out everything is fine and we have no complaints or arguments at all. We always have a great time! For instance today, we were supposed to go to dinner and watch a movie after, and she was going to spend the night at my house. She cancelled again, blamed it on school and how she needed to clean her room and get sleep for work in the morning with her aunt. My problem is I can always tell when she is planning on canceling. She will do the same thing every time...she is really nice to me especially when we text...puts smiley faces in everything and tells me how much she misses me and how she loves me. Then even at school she offered to bring me a slice of pizza after my class. In my head I was like ok, she is probably going to cancel tonight....what do you know and hour later when we met up she explained that she had a lot of homework to do and how her aunt needed her to go into work earlier and she needed sleep. If this is the case then tell me straight up!! I hate when she does this stuff...makes me feel like she is using me or that I'm some sort of idiot. When I try and tell her the things she does wrong she gets mad at me and never considers my opinion...ever I feel like. I have 2 examples of how she almost never considers my opinion. The first one was with this guy from school who she is friends with. They talk pretty frequently and text each other a good amount. They study together for exams at school and do homework together because he is in almost all of her classes. I told her that it makes me a little bit uncomfortable when she talks and texts him all the time because I have never once met him....when we see him at school she just walks past him with me. I asked her if there was anything going on and explained why this was uncomfortable for me and she blew up at me and said I can't believe you would think that!! Then I said well can you at least introduce me to him next time we pass him so I could find out more of who this guy is? Her response was..."why do you need to meet him? He's just a friend, why does it matter? When we text its usually about school stuff" The second example, for instance today I told her that she needs to realize that her actions affect me and it puts strain on our relationship when she does this to me. We will plan great evenings or days and then she cancels. She doesn't do it too often, but she has done it quite a bit the last month or so...give or take 3 or 4 times. Then she knows I get mad and she's like your mad at me...I know it. But she doesn't apologize....Why?? The last thing is, she never cancels plans with her friends. Also I understand that when she is with her friends obviously she wont text me as frequently but at least keep me in the loop somehow. There will be times when she is with her girlfriends and they hang out for hours like 5 or 6 at a time, and I will be lucky if I get 2 texts from her within that time period. I feel that she knows I'm always going to be there with her so she knows she can get away with canceling plans with me sometimes and ignoring me while with her friends. I work around my schedule so much for her and try and be with her as much as I can. She never works with her schedule and we only hang out the days when it's most convenient for her. But yet she tells me all the time she would do anything for me and tells me I'm her everything and how she loves me more than anything...what should I do? I need some advice. What does everyone think? Does she sound self centered? Almost like a narcissist? She does take a lot of selfies on facebook and loves the attention that she gets especially from other guys and basically brags about how many likes she gets on fb. Last thing, I noticed the last few weeks whenever I comment or put something on her wall it's like she ignores me and pays not attention to what I say even if it's something she likes. Then when a friend of hers comments on it she will like their post and respond to them and not to me...like I said at times she feels she doesn't like to put our business out there but seriously?? May be I'm getting frustrated on little things that don't matter, but I guess what I'm trying to ask is if she is worth the time. I love this girl more than anything and I know she loves me too more than anything. Just not sure as to why treats me like this :( Any thoughts or comments? Please! Thanks everyone!!

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