Asklemmy

It's very different for everybody, but here are things that would apply to SOME:

• She might reject "must do now" orders. Instead of saying "Start your homework now and do it until it is finished", change both the start and duration to something manageable. "Hey, you are home! Just relax for 20 minutes, and 5 minutes before dinner starts, get everything for your homework ready on your desk." Starting the actual homework is far less overwhelming, then. And instead of "... until it's done", make a deal like: "You only have to do 12 minutes of the task, but with a challenge: 12 minutes of maximum efficiency and performance!". When it is about cleaning the room, also provide a clear unit of work, such as a time constraint (with stopwatch, never wing it!), or toys only, dirty laundry only, a well-defined section only.
• She might already be the willpower equivalent of a body builder, because she has to do with force of will what other people have done for them, be it the frontal lobe breaking down a task, or handing out dopamine rewards that she does not get. When she starts a task such as homework, she has to face the whole tree of little steps and what could go wrong: Find the backpack, alternative plan for when the math book is not in it, the notebook has half a page left, so she will have to stop in the middle to find the new one (where is it?), ...
• When she is on a productive obsession, such as reading, an instrument, an area of knowledge, let it run its course undisturbed. There might be phases in which everything feels like too much, so these phases are invaluable. Much of her skillset might come from intense obsessions rather than continuous habits.
• Focus on finding a starting point to an overwhelming task, such as point 1: Get the homework ready and in place, then do something else. It might trigger a thing where she WANTS to start immediately, and otherwise, the start will be so much easier.
• Allow her to skip homework when it is too much and write a note for the teacher. E. g. got back home sick, doctor visit on the afternoon, exhausted and unable to finish homework, but did a start. When necessary.