One should always look forward in life and not worry about the past.

Detailed explanation:

Tomorrow holds a promise- the word promise signifies hope or a new begining, tomorrow also signifies something new, so this part of the phrase means- that tomorrow will always bring with it a new hope and a new begining to life .

That yesterday has broken- the word yesterday signifies something the past period of time, and the word broken means something that has been destroyed, or perished , or damaged.

so if you read the entire phrase it wolud mean, that no matter how bad or disturbing ones past has been , one should always look forward in life, because the future has a lot in store for one that would be much better than what one has experienced in the past.

It is a positive thought to motivate someone to forget the worries of the past and move ahead with life

Therefore, don’t dwell on the past because the future is so much brighter