Okay, firstly, if you haven't seen The Wrath of Khan and The Search for Spock, you really should - and not just because this story will make a lot more sense. Pretty much the entire plot of the latter film is Kirk sacrificing everything to save Spock. (The dialogue in italics is taken verbatim from the movie.)
This was written for a prompt on the (new) kink meme: K/S established. Spock is insecure about Kirk's feelings towards him. He's desperate enough to ask Spock Prime about it and Spock Prime lets some events from "The Search for Spock" slip. I'd love if you could have Spock Prime say something like, "Jim stole the Enterprise just so he could rescue me." Bonus points if Spock tells Kirk about it, and Kirk just brushes it off like, "Why are you so surprised? I love you, of course I'd steal the Enterprise to get you back."
Spock takes a moment to centre himself, and then raises his hand and knocks sharply on the door. He is conflicted, has been conflicted for some time now, and so he has come to the only person who may be able to give him some answers. He has only a short wait before the door is opened and his older self looks back at him.
"I wish to speak with you," Spock says.
His counterpart looks puzzled, looking over Spock's shoulder. "Jim is not with you?"
"He is what I want to discuss," Spock explains.
The ambassador nods. "In that case, you should come in." He moves away from the doorway and sits down on a sofa, gesturing Spock to join him. When they are both seated, he asks, "So, what is it that brings you here?"
Spock considers and discards several starting points, before deciding to just blurt it out. "How did you know that he loved you?"
The ambassador's eyebrows shoot up. "Perhaps you should start at the beginning."
Spock takes a breath and begins to explain. The knowledge that he is, in some way, talking only to himself makes it easier to discuss his uncertainty at Jim's actions, and his own tightly held belief that Jim will eventually get bored of him, and move on to someone else; someone who is more, well, human.
When he has finished, his counterpart is staring at him. "You believe that he does not love you as you love him."
"Yes." The emotion of love is still slightly foreign to him, but Spock does not think there is another word to describe his feelings for the captain. "He has never had a successful relationship."
"Neither have you," the ambassador points out, not unkindly. Then he looks away for a moment, and adds very quietly, "You have no idea of the kind of love James Kirk is capable of."
He looks back at Spock and asks, "If you were to name the one thing that is more important to Jim than anything else, what would it be?"
Spock frowns at the abrupt change of subject, but the answer is obvious. "Captaining the Enterprise."
"Ah. So you would say that the Enterprise is more important to Jim than anything else?"
"Yes," Spock responds, feeling as though he is being drawn into a trap.
His counterpart nods again, as though the answer was one he expected. What he says next stuns Spock. "In my timeline, Jim destroyed the Enterprise in an attempt to save my life."
Spock can only stare at him as he continues, "One of our missions together went badly awry. The ship was damaged, and I was the only one who could repair it. Unfortunately, in saving the ship I was dooming myself. It was logical – if I had chosen not to undertake the repairs, the entire crew would have been killed.
"But as you may have guessed, the rather unique circumstances surrounding my death meant that there was a small chance that I could be resurrected." He takes a deep breath. "Jim risked everything, sacrificed everything, on that chance."
"He was successful," Spock notes.
"Yes, he was." His counterpart raises a hand in the traditional meld position. "I can show you, if you wish."
Spock frowns. "You cannot have any memory of that time."
His older self almost smiles. "The memories are not mine."
Spock moves forward slightly, feels his counterpart's fingers on his face and then he is-
Looking up at Sarek and it's taking all of his energy not to just fall apart as he says, "Your son meant more to me than you can know. I'd have given my life if it would have saved his"
Arguing with Admiral Morrow about Genesis and he is practically begging; please, you have to let me do this, "If there's even a chance that Spock has an eternal soul -- then that is my responsibility, as surely as if it were my very own"
Leaving once he realises it's hopeless, and meeting up with Chekov and Sulu. "The word, sir?" "The word is no. I am therefore going anyway"
Rescuing McCoy and stealing the Enterprise, with the help of his friends and he can't believe that they would willingly risk themselves when they don't have to
Setting the self destruct on the Enterprise, and he wishes he didn't have to do this but it's the only way to save Spock and he can't – he can't lose him again
Watching from the surface of Genesis as the Enterprise burns up in the atmosphere – his ship, his ship for so many years, and "My God, Bones, what have I done?"
Seeing David's body and hearing McCoy state that Spock's alive and grief is warring with relief and then he is filled with guilt because his son is dead and he's feeling relief, what kind of father is that?
Standing before Sarek again and despite everything it's worth it. "Your ship, your son." "If I hadn't tried, the cost would have been my soul"
- and then he is drawn out of the meld and his older self says softly, "Do you understand, now?"
Spock nods, his mind still reeling from the sheer depth of emotion he was just shown. The idea that someone could love him that strongly, that deeply, more than anything else in the universe, is unbelievable. Spock barely notices his counterpart stand and leave the room, so focused is he on what he has just seen. He knows, rationally, that the ambassador's Jim Kirk and his own Jim Kirk are not the same man, and yet if his own Jim is capable of a fraction of the devotion that he had seen in that meld, then Spock has deeply underestimated him.
There are many details from the mind-meld that make little sense, and Spock makes a note to ask his older self about them later. But for now he closes his eyes for a moment and allows himself to simply feel.
Once he is back on the ship he heads straight to Jim's quarters.
Jim turns away from the computer as he enters and gives him a brilliant grin. "There you are! Where did you disappear off to?"
"I took the opportunity to visit my counterpart."
"Really? What did you two talk about?"
Spock decides to be honest. "You, actually. Or, more precisely, your counterpart." He swallows. "In their timeline, you destroyed the Enterprise to save his life."
Jim blinks. "Wow." But then his expression clears. "But it must've worked, right?"
"So it was worth it." Jim says this as though there can be no argument.
Spock takes a moment to control the feeling that wells up at this declaration, and thinks again of what he had experienced in the meld. "He believed so."
Jim grins again, and extends two fingers in a Vulcan kiss. Spock meets them with his own and truly believes, for the first time, that this might actually work.