Movies get shot out of order and still make sense. Your project doesn’t have to be built front to back.

A film is assembled rather than performed start to finish, and the audience never knows which scene was shot first. The order you build something in and the order it is experienced in are separate questions. If the opening is stuck, you can make the middle, and the opening often reveals itself later. Front to back is a habit, not a rule.

Take the thing you are stalled on and deliberately do a later piece. Write the last paragraph, record the chorus, paint the background, then leave an obvious gap and come back to it. The piece you make will tell you what the missing one needs to be.