Welcome to PC! Glad you could post.

It sounds like you have an idea of what you boundaries are for this type of friendship. I would say just work on how to be assertive about them.

Worry less about whether or not she is selfish. As an example, you did a great job setting the boundary in your mind about not needing any interuptions during studying and a good job communicating this initially to your friend. However, when she shows up you kind of caved in...which in turn sent a mixed message to your friend that you really didn't mean what you said...so enforce the boundary you told her...

Dig a little into the examples you provided and see if any of these involve the follow through part of boundaries or need a little more assertiveness.

Good luck...