You can preserve momentum without performing at maximum capacity.

Momentum is mostly about continuity rather than intensity. A short, unimpressive session keeps the thread unbroken, keeps the problem loaded in your mind, and keeps tomorrow easy. Maximum output is wonderful when it is available, but treating it as the only thing that counts is how people end up producing nothing for weeks.

Give yourself permission to have a quiet session that no one would be impressed by. It is doing real structural work even when it produces little. Judge the week by whether the thread held, not by any single day.