Everyone has good days and bad days, no one only has good days, and no one only has bad days either. Like it said "Nothing lasts forever" - happiness and sadness is temporary.
We should learn to fully live in every moment of our life without worry about what may come tomorrow and no regret for what already happened in the past.
In the meantime we should be prepared for the unexpected situation may occur by seeing the thing as it is- no attachment to anything.
I know you may think it seems unrealistic, and it is easier said than done, but it is worth!