The poem is about the speaker's struggles of saying goodbye as she approaches death.
Let's look at the first two lines:
Remember me when I am gone away,
Gone far away into the silent land;
Here, the speaker implores the beloved to remember her when she is dead ("gone away") into "the silent land" (death, the tomb).
She knows that after death, there will be no more touching, and no more bittersweet yearning:
When you can no more hold me by the hand,
Nor I half turn to go yet turning stay.
The next two lines reveal her fear that time will equal forgetting, when they have nothing more to look forward to doing together:
Remember me when no more day by day 5
You tell me of our future that you planned:
The speaker understands that this is natural. She does not want to be on the beloved's mind at all times, only that occasionally he think of her:
Only remember me; you understand
It will be late to counsel then or pray.
On the heels of this statement, the speaker seems eager to make herself clear. She does not want him to feel badly if (and when) he does forget her at times. She would rather he be happy, even without her, than to live a life of sadness:
Yet if you should forget me for a while
And afterwards remember, do not grieve: 10
For if the darkness and corruption leave
A vestige of the thoughts that once I had,
Better by far you should forget and smile
Than that you should remember and be sad.