Well, I hate to say it, but once they're in college you kind of have to let go, even if they're still at home. Be there for moral support, etc, but you really can't punish her for bad grades and the like now. It is really common for college freshmen to struggle because college is very different than high school. The work is harder, the expectations are higher, and there usually isn't anyone there to babysit you like before. I learned that way, as did my sister, and we both pulled out of it. But here are some suggestions. First, reconsider letting her have a part time job. My entire life I have had more direction when I had a job, and I went to school full time and worked 30+ hours for most of my college career- I ended up with a BA and five semesters on the Dean's list. Having a job while going to school forces kids to manage their time, and she will also have a little money, which means more freedom. Secondly, let her have more freedom in other areas. Obviously she sounds like a good kid, and it sounds like you trust her. I grew up in a different part of the country, but the second I graduated from high school my parents took away my curfew- but I still had responsibilities. I could stay out until 4 am if I wanted, but I still had to get up at 7 to feed my cows (don't laugh), and I still had to go to work. Or, if she has a cell phone, let her pay the bill herself (once she gets a job). This will make her feel more directed and grown up.
Lastly, you may want to have a very grown up, rational conversation (if you haven't already done this) about what she wants to do with her life and how she wants to get there. I know how valuable education is, but at the same time college doesn't work for everyone. If she has a path and knows what she is interested in doing, life and school become more interesting and she will have more motivation.
Good luck!