Understanding Ds’ sensory profile has been so important for us. We all have eight senses - the obvious 5 (touch, taste, smell, sight, hearing) and then proprioception, introspection, and vestibular. Children who are autistic will crave some senses, and then be incredibly sensitive to others.
If you can limit or tone down the difficult sensory experiences, and give them lots of the sensory experiences that are calming and regulating, then their resilience massively improves.
Typically in school they are bombarded with difficult sensations, and come home wrecked, unable to cope with small issues because they desperately need to recharge.
I’m not picking on your parenting when I say this next bit; I’m just sharing an insight that took me years. When your dc was telling you to stop the hair washing, she was communicating that she was having a difficult experience. You reacted to a child saw as being difficult.
I’m not blaming you at all - you’re following the guidelines. It’s just you’ve been given the wrong manual.
Thought experiment: Imagine for a second that you discovered that instead of shampoo you had been using something else, something mildly acidic that was hurting your dd’s skin. If you checked the label and saw the words “skin irritant” would you have continued the way you did? Would her reaction seem disproportionate?
Neurotypicals have a theory of mind, and essentially this means that they instinctively believe that everyone is having a similar experience of the world as they are. The key to raising a neurodivergent child is being able to understand that this is not always true. A light that doesn’t bother one person, can hurt another person’s eyes.
Once I realised that when ds had a meltdown, he wasn’t being bad - he was having a very bad time, everything changed for us. We’ve rebuilt his trust that I care for him and he isn’t clinging as rigidly to his control because he knows I will try to see his point of view.