Indeed, one of the writer's most important qualities is tenacity. Call it what you will—resolution, determination, or plain old stick-to-itiveness—the writer needs to keep going. There is value, of course, to showing up to put words on the page. At a certain point, though, that dogged perseverance needs to coalesce around a specific project.
Sometimes idea-hopping is good. It exercises creativity and it gives you an opportunity to experiment and play and, hopefully, land on the idea that is a true spark for you. This is the idea that consumes you, the one you feel compelled to explore in depth. This idea has heat and energy. You cannot ignore it. There's no magic to this. It's just a matter of following what intrigues you and finding the story in it.
It may be that the original story idea wasn't all that captivating for you to begin with. An idea with some initial appeal can easily lose energy when held up to the rigors of the writing process. However, if you're routinely abandoning stories-in-progress that once seemed to have that deep pull, you might consider what other issues are at play. Do you struggle with endings? Do you lack confidence in your ability to tell a complete story? Do you better appreciate the first flush of inspiration and feel less thrilled with the work that comes after? Sometimes identifying the concern can help.
Once you think through some of the issues that pertain to you, you may find you're still dealing with story ideas that try to seduce you away from the one you're writing. Your practice of jotting down details of the new story is a good one. It allows you to acknowledge those ideas and to put them somewhere you can return to in order to develop them. That may help eliminate the distraction and sharpen your focus on the story at hand.
You might also revisit the initial inspiration for the story you're writing. Watch the documentary that led to the idea. Reconsider the snippet of a conversation that led to the main conflict of your story. If the interest grew out of research, delve back into that for a bit. This doesn't have to be extensive. Spend some time with the initial inspiration and see what that means for the creative process.
You could also work on freewriting that sinks you fully in the fictional world of the story you're writing. Flesh out a scene from your character's childhood. Write a description of the main setting as it appears at different hours of a 24-hour period. Write a scene from the perspective of a secondary character. These likely won't lead to work you use in a final draft, but they may anchor your mind and imagination more fully in the story.
Finally, don't hesitate to take some time away from the work if you find you're stuck. Sometimes competing ideas come up because you need a break. Following one of those ideas can give you the time and distance from the story that you need in order to enter it anew. The key, though, is to set a date when you will return to the first story to see where things stand. Give it a solid chance. Perhaps even engage in some of the earlier suggestions as a way to get back into the world of the story. Not all stories need to be completed, but give yours the chance it deserves. You may find that lost interest is found.