When writing metered rhyming poetry,
a syllable should not be thrown away.
A wasted phrase amounts to felony
(as lines are few, each word should have its say).
If I may risk reverse humility,
then here’s how I might edit Ms Millay:

I burn the candle at both ends.
It makes a lovely light.
But though the glow so far extends,
it cannot last the night.

The original:
My candle burns at both ends;
It will not last the night;
But ah, my foes, and oh, my friends –
It gives a lovely light!