Always scrutinize yourself. Even if the project went really well, ask “What could I have done differently? What would I do differently I was to do it again now?”.

“This system of self-analysis, self education, continued year after year, did more for me than any other one thing I have ever attempted ” – How to make friends and influence people

This principle can be taken too far though. In the past, when my boss said to me “great job!”, I said “Thanks!” but internally I felt irritated or even mildly offended, because I knew that I had made some (small) mistakes along the way.

If this sounds like you, my advice is to cut yourself a little bit of slack and enjoy a compliment whenever you receive it. Because even if you are giving it your all, there are no absolute guarantees that everything will always stay rosy.

So enjoy the good times, never dwell on the bad times, and always seek to learn from your experience along the way.

John Sonmez actually advises to go to the polar opposite of the perfectionist mentality. The reasoning is that you need to fail in order to improve, so focus on failing multiple times. This is a very novel idea for me, but I love the idea of doing something without putting any pressure on yourself to achieve anything more than finish the task you set for yourself. There is a healthy balance to be struck. I think a combination of low levels of pressure on yourself combined with a smart review and replanning process leads to good results.

Further Reading
Focus on Being Crappy In Order To Be Great