Answer:
At the start of the story, Jekyll is outgoing and healthy, but over the course of the events he slowly gets more ill/weak and becomes antisocial. From his confession we discover that this is because of him loosing control over his other side, Hyde. Since the effect of the potion has changed, Hyde becomes stronger while Jekyll grows weaker. As times goes on, Jekyll's mental and physical state continue to digress. Because of this, Jekyll regrets his actions.