John Steinbeck, author of Grapes of Wrath, Of Mice and Men, and The Winter of Discontent, said to, "Abandon the notion that you ever going to finish." I agree.
They say there is no greater agony than bearing an untold story inside of you. I say the greater agony is finally telling that story but you can't finish it. This is different from writer's block. It's not that you can't write, it's that you can't write that particular story.
I have dealt with this before, in fact, I am still dealing with it. What did I do?
I saved it to a flash drive and then deleted it and anything related to it from computer. I am a firm believer that sometimes you need to get away from a piece work that you can't work on. Only pause, only push it away, do NOT give up on it. If a story has been with you for a long time, it needs to be shared and it deserves to be heard. There is nothing wrong with taking break, but giving up should never be an option.
During the break work on other stories. Don't quit writing. While working on other pieces, your unfinished story might come back to life. Always have a notebook around to write down those ideas so you don't lose them.
So, if you come across this predicament, put your story on the back burner. Each story already comes with a beginning, middle, and ending, it's up to you to fill it out, but let the story guide you. Pause when needed and write with reckless abandon when it wants you to.