"Stop making excuses!" 
I found this guiding principle very transformative. Don't deceive yourself and others with excuses as to why you do or don't do something. Excuses get in the way of you becoming a better you. They distract you from learning from mistakes and keep you stuck in place. I expect the same from the people around me. No excuses! Making mistakes or not doing things is perfectly OK. But justifying it with excuses doesn't allow for progress. In my mid-30s, I pinned a few leitmotifs in mirror writing behind my bathroom mirror so that I could see them in the mirror every morning. 
In my mid-40s I can say - 'Stop making excuses' has the greatest transformative power. 

 Exactly.  Spend your mental power on "How can I?" rather than on "Why I can't."