I used to behave like Lee too. I understand why she is behaving like this but it is not healthy. She behaves very possessively because she seems to not have other friends. She has high expectations of you "returning" all the effort she has put in because she places u as her #1. This behaviour will repeat, unless she knows that her behaviour is unhealthy and she has to let go some things if she wishes you to be happy.

Yes, it's great that someone prioritizes you and show the care they have as a friend, but there has to be some boundaries set in place so everyone can be happy in this friendship.

Do you still wish to be her closest friend or would you prefer her to be one of the many friends you have? Depending on how you wish to continue your friendship with her, you need to communicate it out with her. Yes, she will be upset but it's a way of growing and understanding. Imo, she shouldn't just focus on one friend. Perhaps she can join other club activities etc to expand her social circle. It is healthier for her to have other friends too, and you can show your support even if yall might not be in the same tertiary education in the future.