It helps to put this in perspective. When the engine fails in a helicopter, there is only one "memory" action required to avoid the impending disaster - lowering the collective and entering autorotation. It follows that you must (a) achieve this without decaying the RRPM beyond recovery, and (b) not screwing the main rotor off on the way down with a massive overspeed. The importance of (b) can vary slightly with aircraft type and instructor, but (a) is always a given.
Now all helicopters can increase your level of excitement and provide you with amusing lights and horns during system failures, but very few things will kill you as definitely and assuredly as not entering autorotation correctly following an engine failure. To put this back in perspective for you, your instructor is simply giving you another opportunity to demonstrate that the single most important emergency drill in a helicopter is ingrained in your memory and can be recalled and executed safely and consistently.
Now ask yourself what tiredness has to do with this drill (nothing) and how fast the throttle chop matters (not a bit) and more importantly, could you ever envisage yourself flying in a similar state of fatigue operationally (probably yes). The only score you will get if it happens for real are you versus gravity, so take another chance to understand how critical this is, learn from your instructor's feedback and nail it next time, and next time, and next time and every time.