waning

Becoming weaker or smaller.

my patience is waning

Of the lunar phase: as it shrinks when viewed from the Earth.

the waning moon

The fact or act of becoming less or less intense or present; fading.

the waning of her energy

And boyhood is a summer sun Whose waning is the dreariest one — For all we live to know is known And all we seek to keep hath flown — […]

The fact or act of becoming smaller.

17th century, Joseph Hall, Epistle to Mr. R. B. This earthly moon, the Church, hath her fulls and wanings, and sometimes her eclipses.