"The best time to plant a tree is fifty years ago. The second best time is right now" - essentially don't worry about what you could have done better, or what could have been, make now and the future as good as you can
and
"If you don't fall off occasionally, you're not trying hard enough" - originally told to me in the context of learning to windsurf (I still can't windsurf), but applicable in a lot of areas. This doesn't mean try to fall off, it means failure is a natural part of growth, not something to be shunned