@silhouette,
I had a similar experience when I went to college. Because of the high cost, I worked two jobs (both part time) while attending and was co-chair of a student newsletter.
I was fortunate that when I was having difficulty in a class and met with my professor, he sat down with me and helped me organize my life. He knew I was good at the class, however, my test did not reflect it. So he realized that it was more on my approach with limited time constraints.
He had me put a calendar together where I noted and marked off time for classes, work and then marked down study time. With limited time - you need to actually plan on the time you need to study just as you do with the time you need to physically appear in class.
It forced me to study rather than drag my feet -- I would actually go to the library where I would not be disturbed. My best studying time was Friday late afternoon/early evening before the library closed. Everyone else was getting ready to go out and party and starting the weekend where I would be in an almost empty library. I still went out, but I delayed it by a couple of hours.
It wasn't easy, but it taught me a huge thing -- discipline and prioritizing. Prior to that my grades were average, after that I had above average grades. Only regret was that I didn't learn this earlier.
One thing with the activities - I suggest you put in your study hours first, then add in the activities. Since these are extra it will help you prioritize. You might still be able to do the activities, kind of depends on the time commitment.