For what it's worth:
I'm polishing my ending currently after taking years to write the book. One reason the ending is harder is because there's no more room to go in any creative direction I want to. There's only one way the book can end. Even if there were two or three options, they are still closed, not open ended options.
I didn't know what my ending was going to be as I was writing. I knew what the concept was, the goals, the character's goal. The closer I got the clearer the ending became.
There are a lot of good web pages out there on how to write or structure your endings. I found those very helpful when my critique group said my ending deflated instead of popped. Do take the time to read 'how to write endings' blogs or how to write book chapters on endings. They might give you ideas.
If you are losing interest in the story, that's a problem. You need to rethink that story. But if you are stopping because you have writer's block and it's easier to go do something else, that's a different problem. Read endings in books in the same genre. Read those writers' guides I spoke of in the previous paragraph.