Not Again

I'm thinking of cheating. I've been with my work in progress for the better part of a year and a half. It's a long term project that has crawled along, through the muck, through the briars, to reach 40,000 words. Like most long term relationships, my story and I continue onward, tireless in our pursuit of triumph over adversity, hoping that love will conquer all.
Then, over the weekend, I caught glimpse of a new story idea. Dear God help me, it is the best story idea that I have ever had. Maybe the best story idea I will ever have. Maybe the best story idea that has ever been had by any writer anywhere on this lovely bauble of a globe. All weekend, it played out in my head in brilliant spirals of light. Like a symphony of angels it soared the mountains majesty, dove the deepest ocean trough.
Even now, a few days later, it winks a sultry eye and makes my heart speed. Meanwhile, my WIP waits. It has seen this behavior before, the following down of that other brightly handsome story. Last spring, there was another idea. I met up with it in dark alleyways and anonymous hotel rooms. I worshipped it for 12,000 words.
Until the day came when I woke up to realize that the new WIP wasn't so much better than the old WIP. The jokes, once so clever, had become stale, cliché, older than time. The story wasn't smart or lofty or really much of anything more than a bunch of words grouped together into uninspired sentences and gaping holes of plotline. There was not so much as a single insight.
I went back to my old WIP. It didn't question me. It didn't say I told you so. "Complete me," were the only words I heard. I pledged to honor it, to see it through. We made up. We soared for a while. Until we fought again. Currently, we're not on speaking terms.
And then this new idea. Dear God, this new idea. Will I ever learn?