As summarized in the Wikipedia entry, the script went through many rewrites; apart from the info already available in Memory Alpha, here is Harlan Ellison on the subject, as quoted in the oral histories included in the 2016 book by Mark A. Altman & Edward Gross, The Fifty-Year Mission: The Complete, Uncensored, Unauthorized Oral History of Star Trek - The First 25 Years:
The idea of "City" came from the image of the City on the Edge of Forever, and it was an image of two cities, which is what it says in the script. The City on the Edge of Forever is the city on this planet. It was not a big donut in my script; it was a city. That was a city that was on the edge of time, and it was where all of the winds of time met. That was my original idea. All the winds of time coalesce, and when you go through to the other side, here is this other city which is also on the edge of forever, which is New York during the Depression. They're the mirror image of each other. In that time, all I was concerned about was telling a love story. I made the point that there are some loves that are so great that you would sacrifice your ship, your crew, your friends, your mother, all of time, and everything in defense of this great love.
That's what the story was all about. All of the additional stuff that Gene Roddenberry kept trying to get me to put in, kept taking away from that. The script does not end the way the episode does. Kirk goes for her to save her. At the final moment, by his actions, he says, "Fuck it. I don't care what happens to the ship, the future, and everything else. I can't let her die. I love her," and he starts for her. Spock, who is cold and logical, grabs him and holds him back and she's hit by the truck.
The TV ending, where he closes his eyes and lets her get hit by the truck, is absolutely bullshit. It destroyed the core of what I tried to do. It destroyed the art; it destroyed the drama; it destroyed the extra human tragedy of it.
