too long a sacrifice can make a stone of the heart

Quotes from an address by Nelson Mandela to the Irish Dáil Éireann* in July 1990, just months after his release from prison in South Africa, thanking them for their support:
(bold added by blogger)

"The outstanding Irish poet, William Butler Yeats, has written that too long a sacrifice can make a stone of the heart. He spoke thus because he could feel within himself the pain of the suffering that Irish men and women of conscience had had to endure in centuries of struggle against an unrelenting tyranny.
"But then he also spoke of love, of the love of those whose warm hearts the oppressors sought to turn to stone, the love of their country and people, and, in the end the love of humanity itself.

"For three quarters of a century, under the leadership of the ANC, our own people have themselves confronted a racist tyranny which grew more stubborn with each passing day. It had to be our lot that even as we refused to take up arms to save lives, we still had to bury many martyrs who were shot down or tortured to death simply because they dared to cry freedom.

"The apartheid system has killed countless numbers, not only in our country but throughout Southern Africa. It has condemned to the gallows some of the best sons of our people. It has imprisoned some and driven others into exile. Even those whose only desire was to live, have had their lives cut short because apartheid means the systematic and conscious deprivation and impoverishment of the black millions.

"It could have been that our own hearts turned to stone. It could have been that we inscribed vengeance on our banners of battle and resolved to meet brutality with brutality. But we understood that oppression dehumanises the oppressor as it hurts the oppressed. We understood that to emulate the barbarity of the tyrant would also transform us into savages. We knew that we would sully and degrade our cause if we allowed that it should, at any stage, borrow anything from the practices of the oppressor. We had to refuse that our long sacrifice should make a stone of our hearts.

"We are in struggle because we value life and love all humanity. The liberated South Africa we envision is one in which all our people, both black and white, will be one to the other, brother and sister. We see being born a united South African nation of equal compatriots, enriched by the diversity of the colour and culture of the citizens who make up the whole."

http://www.irishtimes.com/news/politics/address-by-nelson-mandela-to-d%C3%A1il-%C3%A9ireann-1.1422034?page=2

*Note: Dáil Éireann is the lower house, but principal chamber, of the Oireachtas (Irish parliament).