Well yeah, what he said. and then this is what I said, but I'm a slower typer.
My advice is delegate, delegate, delegate.
When you say you're the Co-Chair is that of the whole Kit and Kabootle or just one part of the festival? My wife took over a three day festival five years ago, with the only experience up to that point as being a participant, it was her coordination of the individual groups, that had the experience, that she counted on to get things done. If the co-chair that's abandoning you in three weeks has any kind of contact lists and past volunteers those are your boots on the ground. You just need to command them General.
I'll say that in our hard transition (and I say our, but my wife did all the work, she just told me what to do) to new leadership, we had to learn to do things differently to get things done, but in the end there was more positive feedback, then negative stick-in-the-muds that just said, "Why didn't you do it the way you always do it!"