Follow the energy...
The more energetically charged, passionate, excited, filled with possibility we are, the more energetically charged, passionate, excited and filled with possibilities our writing and writing lives are. A loss of energy is a great time to check in with yourself.
What we desire never comes from pushing. Yes, I appreciate all the examples that prove the opposite is true. However, when we are in the flow of life, there is always enough time, enough support, enough imagination, enough stamina available for whatever we put our minds to.
Keep in mind, Brenda's advice is not permission to go back and rewrite the beginning again.
Instead, give her method at try -- "retrench to a point where you know the story was working and branch off in a new direction" from there.
Let me know how it works for you.
PS--If you decide to retrench, follow the energy withthe 5 Key Scenes in mind.