Sounds like it might be time to move on to a different story!
I don't have much experience, but I think what would help is when beginning a project, write down what gets you excited about doing it in a clear list (like a shopping list - e.g. i love this character, i love the idea of this scene etc.).
Then in the future, if you ever find yourself getting bored/tired with it, consult that list - if those things still excite you, you know there's a problem with the way the story's being told for example or maybe there's not enough of those things / if they don't however - it tells you it's time to shelve the project.
Two things to remember:
* you might find yourself putting a project on the shelf, only to find that new idea or your passion return somewhere down the line. There's a reason people call it shelving something and not throwing it.
* Ideas can be recycled - if you decide to shelve a project, it doesn't mean you can't take a character or scenes from it to be used elsewhere. I remember reading, a famous writer saying that great ideas never gone unused - a great idea might just not be right for the particular project you created it for