Laudator Temporis Acti

Geoffrey Chaucer, Man of Law's Tale 421-427:

O sodein wo, that evere art successour
To worldly blisse, spreind with bitternesse!
Th'ende of the joye of oure worldly labour!
Wo occupieth the fin of oure gladnesse.
Herke this conseil for thy sikernesse:
'Upon thy glade day have in thy minde
The unwar wo or harm that comth bihinde.'

Nevill Coghill's modern English version:

O sudden grief that ever art near neighbour
To worldly bliss! Sprinkled with bitterness
The ends of joy in all our earthly labour!
Grief occupies the goal to which we press.
For your own safety think it is no less.
And in your day of gladness bear in mind
The unknown evil forging on behind!

Robert M. Correale, in Sources and Analogues of the Canterbury Tales, Vol. II (Cambridge: D.S. Brewer, 2005), pp. 294-295, cites Lotario Dei Segni (Pope Innocent III), De Miseria Condicionis Humane 1.21.1-5, 13-14 (tr. Robert Enzer Lewis):

Sudden woe always follows worldly joy, and what begins with gladness ends in sorrow. Worldly happiness is indeed sprinkled with many bitternesses. He knew this who said: "Laughter shall be mingled with sorrow, and mourning takes hold of the ends of joy." ... Sound counsel: "In the day of good things be not unmindful of evils."

Semper mundane letitie tristicia repentina succedit, et quod incipit a gaudio desinit in merore. Mundana quippe felicitas multis amaritudinibus est respersa. Noverat hoc ille qui dixerat: "Risus dolore miscebitur, et extrema gaudii luctus occupat." ... Salubre consilium: "In die bonorum ne immemor sis malorum."