I have a similar problem... or not. (I mean it's not a problem for me.)
I am writing five stories at the moment, but I focus on one at a time. For instance, I can write chapters for one story for six to twelve months. At that time, I typically start getting bored so I change to another story and focus on it.
That way, I avoid getting fed up with a story and I'm much more productive as, when I refocus on a story, I have a fresh overview and can feel excited about working on it. One of those stories is currently reaching 120,000 words (nearly finished).
Of course that's how my mind works and it does not stop me from finishing my tales.
I strongly advise you to follow @what's suggestion (in the comments) of creating a 'slush file' to jot down ideas for other stories, and do not take longer than 1-2 hours jotting those ideas.
If you get too excited with a new idea, it may be best to shelf the story you're working on and try to focus on that new idea for at least three months. Any less than that and you can't fully immerse yourself in your fictional world and characters (which will turn out cardboard-like).
If you can't focus on one idea at a time for less than three months... you've got a serious problem. You need to stop and analyse what is going on: why can't you focus? Is it the story? The characters? Do you have a terribly short attention-span? Whatever it is, identify it and work on it.