This happens to mostly everyone. I'm not sure about the grading system in your university, but 78 on a writing assignment sounds like quite a good achievement already. Some professors just don't dish out great marks no matter how many hours you put into, and 7 hours is a very average amount of time to put in a paper, really.
If talking to your professor doesn't work, then the best advice I have for you is to stop mulling over this and prioritize your time and focus on other subjects to boost your GPA. There's no point in trying to perfect your mark for one class. And university and high school are not comparable, hehe. Doesn't matter how great of a student you are in high school, your GPA is expected to drop once you reach university : P
What year are you in?