All valid responses, thanks guys. Yeah, I'm not really quitting, but I really wish I could. The professor does leave comments on the papers, but I didn't find them helpful. So I asked her for clarification and what I needed to do to improve. She just directed me to the written assignment guidelines and said I didn't meet the requirements. I was pretty sure I did though, so I tried being more thorough on the next assignment. Again, she left comments saying it didn't meet the requirements for a higher grade. I answered the questions thoroughly, so I don't see how I could have. I did outside research and used citations and submitted a properly-formatted list of my resources. I didn't mean to sound like I was complaining or whining, I'm just frustrated and disappointed. I took her feedback and built on it, and I definitely felt like I had improved.
Also, I looked the professor up online last night, and found out that the vast majority of students had given her poor ratings all around on RateMyProfessor. If only I had known who was teaching my course when I signed up. Also also, @FishWings, the papers were all out of 100 points, so 78 is C+ range, and I'm used to anywhere between 85 and 98 for that much effort. Normally I can do a respectable paper in three to four hours. Seven is a new record for me.