In a final message read by an aide on Monday, Sen. John McCain (R-Ariz.) reflected on his years of service to the United States and urged Americans to “not despair of our present difficulties.”

McCain died Saturday at the age of 81, a day after announcing his decision to discontinue his cancer treatment.

“I have often observed that I am the luckiest person on earth,” the late senator wrote in a letter read posthumously by adviser Rick Davis. “I feel that way even now as I prepare for the end of my life.” 

“Like most people, I have regrets,” he added. “But I would not trade a day of my life, in good or bad times, for the best day of anyone else’s.”

McCain also reflected on running against Barack Obama in the 2008 presidential election.

“Ten years ago, I had the privilege to concede defeat in the election for president,” he wrote. “I want to end my farewell to you with the heartfelt faith in Americans that I felt so powerfully that evening.”

The Arizona Republican frequently butted heads with President Donald Trump and established himself as a leading member of the so-called Never Trumpers, a descriptor for conservatives who refused to get behind the president.

Trump infamously mocked McCain’s military record during his presidential campaign in 2015 and suggested McCain wasn’t a “war hero” because he had been “captured” during the Vietnam War.

McCain frequently criticized the president’s policies and tone and famously sunk Trump-backed legislation to repeal and replace the Affordable Care Act. In his final message, McCain urged Americans to look beyond the present political climate.

“Do not despair of our present difficulties but believe always in the promise and greatness of America, because nothing is inevitable here,” he wrote. “Americans never quit. We never surrender. We never hide from history. We make history.”