How do we know that we have given our best at something?
How do you know when your tummy is full? How do you know that you ran to the best of your ability? It’s obvious! It’s like the pain in your leg, and you do not need anybody else to tell you that you have pain in your leg! You do not need anyone else to tell you that you have eaten enough. In the same way, when you put your hundred percent there is a certain contentment that comes from within and you are able to say, 'I have done my best and I cannot do any more'.
But if there is any lack in your action, in your output then the mind says that I should have done little better. Instead of sitting and regretting the past you simply have to move forward, accept the past and take a resolve that next time you will put your hundred present. This very resolution to put hundred percent will make you put that hundred percent.