At the end of the day, have I learned something I never knew before? Have I done something I'd never done before? Am I even a little better at something than I was this morning? Do I understand, appreciate, or accept myself and the world around me a little bit more than I did before? Am I (even a little bit) more comfortable in my skin and ready to face tomorrow?
If I can answer 'yes' to some of these, I'd say it's been a successful day. And some honest reflecting on my answer being 'no' can turn my answer into a 'yes'. Constant improvement. Everyday growth. Direction of motion is more important than speed.