That day had come about through the unimaginable sacrifices of thousands of my people, people whose suffering and courage can never be counted or repaid. I felt that day, as I have on so many other days, that I was simply the sum of all those African patriots who had gone before me. That long and noble line ended and now began again with me. I was pained that I was not able to thank them and that they were not able to see what their sacrifices had wrought.

Explanation:

 

Thousands of Mandela's people had made inconceivable sacrifices on that day, people whose sorrow and courage could never be counted or reimbursed. That day, as on so many others, he felt that he was nothing more than the sum of all those African patriots who had gone before him. That long and noble dynasty came to an end and began again with him. He was hurt that he couldn't thank the patriots and that they couldn't see what their efforts had accomplished.

 

Meanings of difficult words:
 
Sl. No
Words
Meanings
1
ValleyA low area of land between hills or mountains, typically with a river or stream flowing through it
2
CrestThe top of a mountain or hill
3
RipeHaving arrived at the fitting stage or time for a particular action or purpose
4
DownpourA heavy fall of rain
5
ShowerA brief and usually light fall of rain, hail, sleet, or snow
6
IntimatelyIn a way that involves detailed knowledge
Reference:

National Council of Educational Research and Training (2007). First Flight. Nelson Mandela: A Long Walk to Freedom - Nelson Rolihlahla Mandela (pp. 16 - 23). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi..