Yes, the vast majority of the time. ^^; And tbh it's always been weird to me that the opposite is more normalized...
Like...the fact that you feel could have done something better doesn't mean you can't be satisfied. You're a human being with subjective tastes; there's probably ALWAYS going to be something you feel you could do better. So why stress about it? Why behave as if it's an inherent negative...?
You're basically conditioning yourself to be eternally disappointed in everything you do...and believe it or not, an automatic shame response may not be the best way to motivate yourself to keep growing and learning. =/
It's a little of both, I think? Like, it depends on the situation, or the specific strategy you're trying to encourage.
If you are constantly running out of time to do things the way you want; if a deadline feels more like a prison than a simple limit, then by all means, start giving yourself more time. Pace yourself better, leave yourself space to relax and reflect instead of working 24/7. Especially if your deadlines are self-imposed.
But on the other hand, if you tend to overthink and second-guess your decisions, working on things forever unless you have a specific stopping point...forcing yourself to meet a deadline might be the better course of action. And if you're having trouble meeting deadlines, you may want to adjust your expectations or lessen the scope of your projects, rather than moving your timeframe.
By...applying them? ^^; I don't really understand the 'how' in this question...if you want to avoid a mistake in the future, (a) identify it, (b) devise a strategy for avoiding it, and (c) employ that strategy next time you're in a similar situation. It's really as simple as that...
If you're having issues with (a), you could try getting input from other artists. Explain your workflow, and ask for their opinions. If they see anything horribly wrong with your process, they'll tell you.
If you're having issues with (b), other artists can help with that too. But you can also figure it out by analyzing the issue at hand. Is it a skill issue? Maybe you need to start doing practice on the side; or reworking your goals so that particular skill is no longer needed. Is it a preparation issue? Then get prepared; gather the references and resources you need, and figure out a way to make them easy to access. Also, the right strategy to help you avoid a mistake will largely depend on your understanding of your own habits, needs, and mannerisms, so you'll need to develop some self-awareness if you haven't already.
Finally, if you're having issues with (c)...usually it's due to an external factor. Maybe a non-negotiable (and likely unhealthy) deadline is forcing you to rush through things whether you want to or not. Maybe some aspect of your environment (noise, interruptions, physical discomfort) is making it difficult for you to focus and think things through. (a) and (b) aren't going to mean much without (c), so you should focus your efforts on resolving the outside issue, at least as much as you can.