I agree with Mare. Collabing is actually a lot harder to finish than not, because it's easier to come up with excuses. Plus, it might be hard to find someone who wants to jump in in the middle. It would be too hard to keep it going in the same style and tone and you probably already have something set in your mind for the rest of it. It would be hard to get that idea across. Collabing can be a fin experience though, and if you're just looking for a new approach for writing, who knows it could work for you. But I'd definitely start from the beginning and come up with a story idea together after you've found a writing partner. Don't expect that that will necessarily help you to finish though, sadly, that just usually not the case.