Sonnet CCXXIX: "This is a sorry ending to a thing"

This is a sorry ending to a thing
We once called love, in our fatuity,
Boasting that nothing worthier could be,
Beyond the limit of its charmed ring!
Was it for this I set myself to sing,
Not as a poet, as a devotee;
Making a marvel of what others see
As common stuff, through my imagining?
Today I saw thee, blushing at thy name,
Stealing from shadow unto shadow, spread,
Like mercy's pall, around thy lustrous head;
And all thy praise was blurred with one great blame,
And all thy beauty was a snare to dread,
And all of love that lasted was its shame.