Oye oye oye, I hate it when that happens. (happened to me too).
Alllrighty here I go, and I hope this could shed light on your situation:
When a relationship starts its always fun and games, but after time passing things begin to settle down and become almost routine like how they used to be + time with that special person. For you, it's probably settled already, and you're back into the groove of your obligations and your family loyalty. your girlfriend, unfortunatly, has probably not settled back.
It appears you have a busy schedule, and she doesn't have much to do. That could be a problem. Her spare time is used on you, all the time, because she has enough spare time to spread it out. You (assumingly) do not have enough to use a majority of it on her, because you would like to see your family, take care of yourself, and take care of college work.
7 hours of a day is more or less half of the time you're awake. Make it clear that you are spending as much time with her as you can, but you have priorities that are just as important in different circumstances.
you are not being selfish, you are taking care of yourself. She is the one being selfish by making you go change everything so you can see her. Tell her that- your schedule has to be changed all the time for her to see him and its a lot harder for you because you have more to do. If she has all this free time, can't she change hers around sometimes? It's only fair.
Theres nothing wrong with spending time with yourself and relaxing. If she decided to get pissy and call you a lier and a dick, and then question your devotion to her, I think that's a damn good reason to get mad.
You're not lieing if you're tired and explain it.
You're not a dick if you spend so much time with her and you need a break once in a while. Seriously, how often can you be with someone and NOT get bored?
You love her, and she denies it? she needs to stop being rash and stop questioning you over something so trivial- I think SHE is the one who needs to be questioned if she keeps that up.
I think you're not selfish, and that she is.
I think that you have a solid grip on what you have/need to do, and she doesnt.
But I do think that after discussing and talking about it, you both will be okay.