That grief has now been cataloged by historian Ellen Fitzpatrick in a new book, "Letters to Jackie," a first-ever compilation of some of the 1.5 million condolence letters the first lady received after Kennedy's death.
Most of the letters were originally destroyed by the National Archives, which felt it would not have enough room to store them. Fitzpatrick combed through more than 15,000 of the remaining letters at the JFK Presidential Library and Museum in Boston, to choose 250 for the book.
"The letters that were most compelling to me were ones that encapsulated some sense of Kennedy as a president, or it was someone who had something very powerful to say about the day of the assassination … or someone who talked about an experience with grief in their own life," said Fitzpatrick.
There was also great diversity among the letter writers.
"I am but a humble postman," wrote Henry Gonzales. "Please try to find it in your heart that we Texans of Mexican origin love all of you."
Martha Ross, the 74-year-old daughter of a slave, wrote: "I am a colord lady but he seam clost to me as my own and he was apart of all Americains..."
Her great-grandson, Winston Lucky, said he remembers as a boy watching her grieve.
"She took the death of the president really hard," he said. "She was crazy about the president. She thought he was a great man and that he was going to do great things for the African-American community."
The book also contains letters written by children, like eight-year-old Kevin Radell.
"I know you should forgive your enemies, but it is hard to forgive Lee Oswald," he wrote.
Today, Radell remembers the period as a "terrifying time." Kennedy, he says, "was such a protector of the nation and such a leader and we all loved him."
Some letters were strikingly prescient. One, signed simply by "A Negro Who beleave in God," blessed the first family, and added: "In the next Forty to Forty-five Year A Negro from Louisiana will be come President of the United States."
For Fitzpatrick, the letters show not only the raw emotion of the time, but the way that Kennedy "had incorporated this message of hope, of vitality, of change, of possibility," with his young family.
"It was a sense of here was a couple, a family who had everything. They were sitting on top of the world. And in an instant it was gone." That is what made the death so devastating, she said.
"It was a loss of innocence, it was a terrible encounter with irrational violence of the kind that I think Americans today are much more used, to, unfortunately."