So what would I tell myself back when DD was two and three?
4. Don't-- ever-- in educating this kind of child-- make them REPEATEDLY demonstrate mastery. What-- are you worried that it was a FLUKE?? This undermines your child's intellectual autonomy and confidence, and it also makes the oppositional tendencies WAY worse. If they master a skill and show you that they truly have mastery--
then let it go. Quit "teaching" what they have mastery of. If you see REFUSAL, consider that you are perhaps insisting on instruction of a domain where the child has already attained mastery. I cannot
begin to explain how much grief this one bit of advice would have saved me and my daughter when she was 4-9yo. They jump out of material with full mastery-- and you won't always see it progressing the way that one assumes will happen. Sometimes it's just as though it "sets up" overnight and emerges whole. Don't poke and prod and kick the tires-- just accept it and move on. For both your sakes.

5. One more-- this is one that educators will argue-argue-argue with, I'm aware... but repetition generally does NOTHING for this kind of child as a learner. Either an approach works for them-- or it doesn't. If they are having trouble grasping something after you've gone through it once (or at most three times) using the same approach... the approach isn't working. They don't NEED 'time for it to sink in.' Mostly. In this kind of instance, one of two things is at work: a) the child's developmental arc just isn't quite ready to support whatever-it-is, and no amount of instruction is going to make it so, or b) a different approach is needed. Period. The other alternative (and we've seen this too) is that the child has retained the message and will work on it mentally and in private (maybe even subconsciously?) until it emerges whole (as noted above). DO NOT BEAT DEAD HORSES in offering instruction to a child like this. DO NOT.