I have been finding some ways of overcoming this challenge. The most effective way I've found is to phase the sketch into three distinct passes.
First pass - plot the stars and their relative brightnesses.
Second pass - sketch the rough shape and extent of the object. By this time this might be by memory, because the dimmest detail is already being washed from view by the light required by sketching.
Third pass, it's lights out! This is the time where I am really straining for every bit of detail possible, making mental note of any mottling, dark lanes, areas with brightening, and limits of extent. Then the light comes back on and adjustments are made to the sketch. If it's just a faint fuzzy galaxy, there's usually not much detail to add once the general shape and core brightening gradient is captured. But -- some objects require a repeat of this step many many times if there's a high level of complexity visible (like the Orion Nebula or Andromeda Galaxy).
When using a filter on nebulae, I find it useful to sketch the star field first with no filter, then put the filter on for the second & third passes to capture shape and extent.
I also find it very necessary to fine tune yet again once back in daylight, as soon as possible after the session. No later than next day so that you still remember what the intent was.