Imagination and collaborative fiction-writing

So late this afternoon Kate Schaefer of the Clarion West Write-a-thon sennt out an email announcing that Michael Swanwick has started a round-robin story on the Write-a-thon forum. I decided to check it out and saw that Michael contributed one sentence, Ruth Nestvold another--and Eileen Gunn a whole slew of sentences. Because Eileen dragged Richard Nixon's name into it, I couldn't resist making a contribution myself. When I visited the site after dinner, I found that Eileen had continued on from where I'd left off. So of course I continued on from where she left off. But after I'd posted my new paragraphs, I discovered that someone else (who is identified only by login handle and not by name) had also posted an addition, which had been added before mine.

How disorienting! Imagination is such a powerful thing. Even though this is a barely-begun story, I'd so fully and vividly imagined the world being created through our opening sentences that I'm now feeling completely thrown out of the story (particularly since the other person's addition constitutes a sharp bifurcation from the direction I'd been imagining). I'd never realized just how thoroughly invested I get into a fictional creation, once I've put words down on the page, even for something so evanescent as a round-robin story. I simply can't overwrite the reality of the words I've already written (which, it seems, is substantially different from adapting to new directions the story might take). I've never done this before, but I'm thinking I'm not really cut out for this kind of collaborative fiction writing...

PS You might say that I have no business just now investing my imagination in anything but the novel I'm trying to finish. And you'd be right.