Leaving an abusive relationship is never easy, and it takes considerable emotional strength to get out.
Traumatic bonding is the strong emotional attachment between the abuser and the victim. It's what makes it harder for a victim to walk away from an abusive relationship than from a healthy one. When an abuser demonstrates intermittent kindness or compassion toward the abusee, they are flooded with a sense of gratitude and appreciation for this mercy. These positive feelings make them more attached to the abuser, and they interpret the lack of abuse during these periods as proof that the abuser is good and truly loves them. The abusive cycle then becomes associated with love, making it much more difficult for the abusee to leave.
An abuser will often make temporary changes when confronted with the possibility that the abusee will leave. However, these changes are rarely sustainable because of the personality structure that allows an abuser to mistreat others and avoid taking responsibility in the first place. In order for real change to occur, there must be true accountability (not just words), real remorse (which requires empathy, something that is typically lacking), and ongoing effort to change, which is a very difficult and lengthy process.
Since the abusee is invested in the relationship's outcome, they cling to the hope of the abuser truly "changing." This false hope then bleeds into denial, which provides temporary comfort because it spares the abusee the struggle and difficulty of ending the relationship. Some people feel sorry for their abuser or worry what will happen if they leave. An abuser capitalizes on this, making it that much harder to leave. The more time one spends in a relationship with an abuser, the more worn down they become.