What does Nelson Mandela taught us about 'forgiveness' ?

One of the most popular autobiography "Long walk to freedom: Nelson Mandela",  tough central theme of the book is 'freedom from racism' it also taught us many life lessons and we can acquire the most important real life virtues from this book i.e., Compassion, Kindness, Courage, Forgiveness, Fight against violence etc., 

Before Nelson Mandela left prison, he said "As I stand before the door to my freedom, I  realize that if I  do not leave my pain, anger and bitterness behind me, I will still be in prison".

Self imprisonment is worse than that imposed. How many are in self inflicted  pain today for lack of forgiveness. How many of us have imprisoned ourselves inside the walls of anger and bitterness. 

Holding grudges does not make you strong, it makes you bitter. "Forgiveness does not make you weak, it sets you free."