Meaning of "The most I ever did for you, was to outlive you, / But that is much"

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:

• Lifelong partners often express a wish: let me be the first to die, meaning that life would not be worth living if the other partner were no longer around.

• If a poet (Millay) outlives a friend or a lover, s/he can perform the service of memorializing the deceased — although this may come at a high price in grief and loneliness.

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):

Guilt magnified her grief. Of the three daughters, she was always the one who assumed responsibility. She jotted lines she would never publish:

At least, my dear
You did not have to live to see me die.

Considering now how many things I did that must have caused you pain,
Sweating at certain memories, blushing dark blood, unable
To gather home my scattered thoughts
. . . . . . . . . . . . . . . . . . . .
You kept no books against me! In my own hand
Are written down the sum and the crude items of my inadequacy.
The most I ever did for you was to outlive you.

(In 1954, Norma would revise the thirty-line fragment and publish it posthumously in such a form that it appears to be addressed to Edna's late husband. But it was written in 1931, for Cora.)

(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.