What’s the best way for a 18M to break up with a 21F?

I'm an 18-year-old guy and I want to end my relationship with a 21-year-old girl I've been dating for the past two months. She's very romantic and sexually active, which I struggle to keep up with. As I’m studying Law, my grades during my first semester have suffered because I've been spending too much time on dates and being intimate with her. This is my first serious relationship, and I've found it difficult to balance my studies, exercise, and time with her. I tried to break up with her a few weeks ago, but since we attend the same college and share a friend group, I wanted to handle it delicately and suggested we could remain friends. Unfortunately, she seemed to feel embarrassed by my attempt to 'friend-zone' her and responded by being quite unkind. The next morning, we agreed to stay together and try to find a balance between studying and our relationship. Initially, I thought we could make it work, but now that the new semester is starting, I'm seriously worried about my grades slipping again. While I'm just starting out in college, she’s almost finished with her program and has already secured good grades, allowing her to leave at the end of the year. We're scheduled to meet on the day we both return to college in a few days. What's the best way to end things with her? Should I do it that evening, or would it be wiser to gradually distance myself to help her adjust and prepare for the inevitable?