I have been writing a series of stories for about fifteen years now. It started as a single story but it bloomed into something completely different. I won't go into specifics but it's a very complex, fluid story and I'm having a hard time sitting down to begin it.
The original version was with one main character, we'll call him John. Over time John was fleshed out over the course of three different full-length iterative versions of this book (which I have since scrapped or thrown out altogether), but now his story has transmuted into something wonderful, but also completely different.
The complexity is a bit much for me to take in to the point where I've made a rather sizeable reference file so I can keep track of the massive amounts of information. But this information is always changing, the story is liquid and impossible to hold down.
With brevity, my problem is this: the story changes so much that I can't seem to find a good time to just wrestle it to the ground and get it going. I fear that I will write it, it'll go somewhere, then I'll come up with a much better idea (which I have a few times now) and sorely regret the finalized version. A lot of these ideas are...I don't know, character-specific so I couldn't just use the ideas in a different story.
I've tried writing a master outline for the entire series of books, it's looking to be about six novels total and that is kind of helping, but that paranoia is still there. I suppose this problem wouldn't be as horrible if it were only to be one story, one novel, but that just isn't the case. I need to write this before it completely devours me and I go insane.

Does anyone have experience with this problem? Any advice would be very helpful.