How do you know today is the best day you would ever experience? My bet is that you do not. Along could come an even better day, one in which everything is magical and your feet can’t touch the ground. Or maybe the rest of your life becomes a blur, with no thrills or excitements.
Live the day.
How do you know the idea you have — and keep procrastinating on — would ever be fully formed? My bet is that you do not. Ideas rarely come in isolation and are often a melting pot of personal experience, environmental triggers, external learnings and natural aptitudes. The idea as it is may be all there would be, for the next year or two. Or it may evolve, as you give it more thought over the next couple of days.
Test ideas fast. If they fail, you at least know one more thing that does not work.
How do you know that the work you have done is the best you would ever do? My bet is that you do not. Enjoy the outcomes but do not dwell too long in the feeling of satisfaction. There is little upside in accepting that you have reached the pinnacle. If the mind says “this is good, I can do great”, an incentive exists to apply yourself more. Thinking like a beginner helps to keep the mind always curious and growing.
Keep growing. To stay stagnant is to die.