I think that it would be a good idea to first decrease the size of your project. Retool your story to work with the levels you have made, and the levels you know you can finish. Add cutscenes if necessary. Split it into episodes if you can. There is a reason why very few large projects ever get released, and it's the exact thing you are running into right now.

Maybe you'll find someone to collaborate with, but group projects don't usually turn out well unless someone is taking a strong lead. You should work with what you have instead.

The secret to releasing a good single player episode is to be able to be content in releasing something that falls short of your original big ideas. It is necessary to compromise and retool things so that managing the project becomes realistic. This is how things get released. Either that, or just start off with small plans in the first place. There's nothing wrong with doing this: my first four single player episodes were all released as smaller than I originally planned them to be (in fact, the first two are almost entirely made up of recycled material with a story tacked on). The fifth one is a bit of an exception since it's huge and actually bigger than I originally planned for it to be, but I have no idea how I did that. In any case, as long as you don't go the tomb rabbit 2 route (where everything is disconnected) it's fine.