Well, it might involve strategic planning and careful manipulation of circumstances. Maybe the character needs to find allies or expose the emperor's weaknesses.
You could show your firm attitude and make it clear that you have no intention of reconciling. Maybe avoid giving him any false hopes or ambiguous signals.
There are many possible reasons. Maybe the childhood friend had always secretly loved the protagonist and couldn't handle any competition. Or, some trauma or insecurity could drive them to be obsessive. Another possibility is that misunderstandings or external pressures played a role.
The ending of Girlfriend Girlfriend manga is quite complex and bittersweet. The characters go through various challenges and their relationships evolve in unexpected ways.