Robert McKee: I'll tell you a secret. The last act makes a film. Wow them in the end, and you got a hit. You can have flaws, problems, but wow them in the end, and you've got a hit. Find an ending, but don't cheat, and don't you dare bring in a deus ex machina. Your characters must change, and the change must come from them. Do that, and you'll be fine.