Mmm. I think those are two very different questions, and actually, that reframing the second may help with the first. As a child myself, I used to feel I had to put my best effort into everything. It was stultifying, because actually, you can't - there aren't enough hours in the day, and anyway, some tasks are just not weighty enough to take it. I ended up overthinking everything and being stressed out, and I didn't learn to prioritise and take responsibility for choices. I still struggle with this to some extent (I'm the one with dust all over my house because I haven't time to dust properly - really, it would be better to do what I can do in 5 mins sometimes! Sometimes a job worth doing is worth doing badly :-)
With DS, I'm trying to help him commit to doing his best sometimes, but at the same time, to recognise "good enough" in context for other things. I don't feel I have a great answer for how to do this in practice, though - I haven't got far beyond lecturing about it. School gets each child to choose three "targets" each term which are the areas they are especially going to focus on, and I think that's helpful.
So on the one hand, I think that recognising that effort in is a knob that has many positions may actually help with turning it all the way up sometimes and seeing what happens; on the other hand, one way into a challenging activity is actually to do it the first time *without* committing to maximal effort. I forget who it was, here or elsewhere, but someone tells their child that the aim for a first go through of a piano piece is to play it terribly badly (but to play it!) DS didn't bite when I tried that particular thing with him, but I like the idea.
