But we live through the fine days without noticing them; only when we fall on evil ones do we wish to have back the former. With sour faces we let a thousand bright and pleasant hours slip by unenjoyed and afterwards vainly sigh for their return when times are trying and depressing. Instead of this, we should cherish every present moment that is bearable, even the most ordinary, which with such indifference we now let slip by, and even with impatience push on.

[Aber wir verleben unsre schönen Tage, ohne sie zu bemerken: erst wann die schlimmen kommen, wünschen wir jene zurück. Tausend heitere, angenehme Stunden lassen wir, mit verdrießlichem Gesicht, ungenossen an uns vorüberziehn, um nachher, zur trüben Zeit, mit vergeblicher Sehnsucht ihnen nachzuseufzen. Statt dessen sollten wir jede erträgliche Gegenwart, auch die alltägliche, welche wir jetzt so gleichgültig vorüberziehn lassen, und wohl gar noch ungeduldig nachschieben.]

Arthur Schopenhauer (1788-1860) German philosopher

Parerga and Paralipomena, Vol. 1, “Aphorisms on the Wisdom of Life [Aphorismen zur Lebensweisheit],” ch. 5 “Counsels and Maxims [Paränesen und Maximen],” § 2.5 (1851) [tr. Payne (1974)]
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(Source (German)). Alternate translation:

But we live through our days of happiness without noticing them; it is only when evil comes upon us that we wish them back. A thousand gay and pleasant hours are wasted in ill-humor; we let them slip by unenjoyed, and sigh for them in vain when the sky is overcast. Those present moments that are bearable, be they never so trite and common, -- passed by in indifference, or, it may be, impatiently pushed away.
[tr. Saunders (1890)]