Fortuna vitrea est; tum cum splendet frangitur - Fortune is glass; just when it gleams brightest it shatters
Fortuna caeca est - Fortune is blind. (Cicero)
Fortunatus sum! Pila mea de gramine horrido modo in pratum lene recta volvit! - Isn't that lucky! My ball just rolled out of the rough and onto the fairway!

Frangar non flectar

I am broken, I am not deflected

Frater, ave atque vale - Brother, hello and good-bye. (Catullus)
Fronti nulla fides - No reliance can be placed on appearance. (don't judge a book by its cover)
Frustra laborant quotquot se calculationibus fatigant pro inventione quadraturae circuli - Futile is the labor of those who fatigue themselves with calculations to square the circle. (Michael Stifel, 1544)
Fugit hora - The hour flies
Fugit inreparabile tempus - Irretrievable time flies. (Virgil)
Functus officio - Having discharged his duty and thus ceased to have any authority over a matter