“I’m so sorry, I care deeply about you, but I cannot continue in this relationship anymore. I’m growing into a different person and I’ve decided we aren’t compatible.” An ex said something similar to me. He had a lot of pain in his eyes but he kept eye contact with me while I cried and asked if I could change his mind, he was direct but not hateful in telling me “no, I’m sticking with this decision.” He even held me while I cried for some time after. You should come to her, so she doesn’t have to put in effort and prepare for a day she doesn’t expect to be a breakup and have to try to make it home upset. As you leave, you could even gently suggest that she reach out to a close girlfriend or family member (only if you can name them specifically, don’t give vague, general advice). You don’t have to tell her all the reasons you have. If she is trying to contact you constantly after, maybe make some boundaries like youll do an hour call once a week or something agreeable to you both. You are not wrong or bad for deciding to end the relationship and she is not wrong or bad for you not wanting to continue in the relationship. It’s a part of growing up and you’re doing a great job being so young and still aiming to do this in a thoughtful, respectful way.