"There are 2 days in a week about which and upon which i never worry. two carefree days kept secretly free from fear and worries. the two days are yesterday and tomorrow" - Robert.J.Burette
Because these are the only two days which are not in your hands. The "Yesterday" could be changed when it was in our hands during our past but since we couldn't do anything about it, we just state it as 'bad fate'. There in nothing called as fate.
'What you sow, so you reap' . Each person's ball is in his court and never ever blame others. Thinking of the past is a real acid test and of course you have to learn some experience from it.
Let's say that you are climbing up a mountain and you have reached half way. Now the mountain has become very steep and you feel very difficult to climb up. Will you return on the same way you came or you will move ahead? I guess no.
Or let's say you are an athlete and running so fast, you suddenly feel very tired. So will you stop running and return on the same track you had crossed? Never. So just erase the harsh memories of whatever happened in the past. Today is the only day which is in our control.
'Act-Act in living present' - This was a line from the poem 'Psalms of life' . So just relax and forget about everything that happened yesterday. Because thinking of the past is the archilles' heel of achieving above board in future.