Write it on your heart that every day is the best day in the year. He is rich who owns the day, and no one owns the day who allows it to be invaded with fret and anxiety. Finish every day and be done with it. You have done what you could. Some blunders and absurdities, no doubt crept in. Forget them as soon as you can, tomorrow is a new day; begin it well and serenely, with too high a spirit to be cumbered with your old nonsense. This new day is too dear, with its hopes and invitations, to waste a moment on the yesterdays.

Ralph Waldo Emerson (1803-1882) American essayist, lecturer, poet

(Spurious)

This widely spread inspirational quotation is actually a piecing-together of multiple phrases from different sources, some not even Emerson. It includes bits from his essay "Works and Days" (here and here), observations on Emerson by Lillian Whiting, and fragments from a letter by him to his daughter Ellen.

The result sounds much like Emerson, but would require more ellipses than text to qualify as a quotation of him. It would be a great candidate for an AI "quotation" except that references to it can be found back in the 2010s, so it is almost certainly of human origin.

More detailed discussion: poetry - From which book or essay are these words by Ralph Waldo Emerson? "Write it on your heart that every day is the best day in the year..." - Literature Stack Exchange.