(covers information from several alternate timelines)
"You don't know how many times I wished I was."
Following the death of Captain Garrovick and most of the crew from the USS Farragut by a dikironium cloud creature, Kirk felt terrible guilt over his captain's death. Years later, Kirk admitted that "I could have killed that thing if I'd fired soon enough the first time"; however, Doctor Leonard McCoy attempted to dispel Kirk's guilt, which was bordering on obsession, assuring him, "You don't know that, Jim." (TOS: "Obsession")
In an alternate timeline, when Chakotay and Harry Kim successfully returned to Earth in 2375, via slipstream aboard the Delta Flyer, at the cost of the loss of USS Voyager, Kim experienced the profound effects of survivor guilt, also to the point of obsession. Kim's counselors at Starfleet Headquarters told him "You must learn to accept the fact that you lived. Embrace life, move forward." (VOY: "Timeless")
In 2390, while in discussion with The Doctor, following his reactivation, about the occurrences of the past fifteen, The Doctor pointed out that "I suppose it must have been difficult, with all your friends and colleagues left behind." Kim caught on to what The Doctor was getting at, he recalled the diagnosis, but still did all he could to undo the wrong he did. Upon finally correcting his mistake Kim left a message to his past self explaining, "Fifteen years ago, I made a mistake and one hundred and fifty people died. I've spent every day since then regretting that mistake, but if you're watching this right now, that means all of that has changed. You owe me one." (VOY: "Timeless")