What did the poet mean by the following lines?
The most I ever did for you, was to outlive you,
But that is much.
— Edna St. Vincent Millay
I am not able to understand the meaning completely, and the context in which this was written.
I see two meanings here:
Edit: (with thanks to @Peter Shor) From What Lips My Lips Have Kissed: The Loves ond Love Poems of Edna St. Vincent Millay (Edna's mother, Cora, has just died):
(Norma was Edna's sister, and acted as her literary executor after her death.)
So, although the published version of the poem may have a slightly different emphasis - and I suppose one might speculate as to whether she really loved her husband or not - as she originally wrote these lines, she meant "I let you down, I never did enough for you, but you always forgave me. And now, the most I can do for you is to go on living without you."
What the poet is saying is, that the only thing, the speaker did, was to outlive the other person, and even the act of outliving that person was difficult.
Being the one left means that you are the one who must mourn and experience great sorrow. I personally love my wife enough to be the one who must bear this rather than her.