Secret service agent Clint Hill risked his life when he jumped out of a moving car in an attempt to save President Kennedy and First Lady Jackie Kennedy, who came under fire during a 1963 presidential parade in Dallas, Texas.
Before Mr Hill was able to shield the couple, a bullet pierced through President Kennedy’s head, killing him instantly.
On the 55th anniversary of the assassination, Mr Hill, 86, revealed how he “should have been faster” and continues to blame himself for the president’s death.
Speaking to the Sun Online, the former secret service agent described the bloody sight he witnessed after President Kennedy was shot, and how the gruesome scene continues to haunt him to this day.
He said: “One thing that I’ve never been able to erase from my mind is being on the back of the car looking down at the president, who was lying with his face in Mrs Kennedy’s lap.
“The right side of his face is up and I can see that his eyes are fixed. There’s blood everywhere.
“I can see the gunshot wound. In the room that’s in the skull I can see that there is no more brain matter left.
“That is something I could never, and have never been able to, erase from my mind.”
The 35th President of the United States was shot dead in Dallas, Texas on 22 November in 1963 by Lee Harvey Oswald, a former marine.
Oswald fired a bolt-action rifle three times out of a sixth floor window, targeting President Kennedy and his wife.
Upon hearing gunshots, Mr Hill leapt out of a nearby car to form a human shield around the president and his wife – but he was a few seconds too late.
A bullet pierced Mr Kennedy, spewing his blood and brain matter all over his car seat – an image Mr Hill says he cannot erase from his memory.
Mr Hill earned an award of bravery for his heroic act, but said he personally felt he failed to do his job and protect the president.
Speaking to the Sun Online, he said: “I think I should have been faster.
“My job was to protect them and I was unable to do that.
“If I had been slightly faster I may have been able to prevent the president’s fatal wound and that has bothered me ever since. It always will – I’m sure.”
Mr Hill described how he suffered from PTSD and spent years addicted to alcohol, and how overwhelming feelings of guilt led him to seclusion.
He said: “I self medicated with alcohol during that period of time. I just didn’t care about anything and I didn’t want to have any contact with anybody.”
“Friends would come by and I wouldn’t even acknowledge that they were there. I just ignored everything.”
He added: “I wasn’t as close to my children as I should have been. They grew up pretty much without a father – their mother raised them.
“I was gone 90 percent of the time. It’s only in the last 10 or 15 years that my two sons and I are much closer than we’ve ever been, which is wonderful for me.”