In singing (or just about any endeavor), one must be observant of what goes well and what goes poorly. Arguably, most of this critical observation should be done soon after the activity, not so much during the creative process.
Determine why some things went well. Be grateful for them, and re-create these circumstance and potential choices again the next time.
Determine why some other things did not go well. Rather than use these failure-contributing events or choices as excuses — thus letting yourself “off the hook” — get right back on the hook. Next time, create different background events and/or preparation, so that you can make more desirable choices. In other words, learn from mistakes — including those you did not commit on purpose, or were not even aware of.
A mistake is a choice you make that does not lead to your best outcome; that’s why we are always works in progress, learning which choices to make and which ones to avoid. The person who accepts responsibility accepts the power to affect change.