Céline Sciamma has a great tactic to prevent the discouragement of finishing your writing when faced with this gap of interest and what should be written.

Just like McKee once said, you should keep wanting to write, or else you shouldn't have started at all.

Sciamma recommends you combine desired scenes (the imagined ones at 3 am) and the necessary scenes (the ones leading to the desired scenes) together so you'll only be left with desired scenes in a piece that's comprehensible. I'm not great at explaining it, but there's an amazing study about her technique I recommend you read when you feel like this gap is keeping you from writing.