First of all, a symbol is defined as...
...an object, picture, written word, sound, or particular mark that represents something else by association...
The most significant symbol in Roo Borson's "After Death" is the chair. At the start of this short poem, Borson explains that she has made it a symbol:
Seeing that there's no other way,
I turn his absence into a chair.
The author notes that she has no other way to cope for the loss of her loved one, but instead of coming into her home aware of its emptiness without him, she pretends that the emptiness is a common chair, something much easier to deal with than loss.
The author also writes:
I can sit in it,
gaze out through the window.
I believe that this means that she wraps herself in the loss of this man, looking out the window, perhaps feeling separated from the world at times, and tied to his memory for she cannot find a way to get past her pain.
The speaker then says:
And I can return then with my useless love,
because the chair is there.
Her love, she says, is useless because she cannot share it with him. However, in saying this, she notes that she still loves him. When she comes home, she finds some peace as she rests in his memory and in the emptiness of the house. Perhaps this is because while he is not with her physically, he is closer to her when she is at home, and she chooses to wrap herself in the emptiness, for it is all she has left of him.
Two sources note that a chair is symbolic of rest. One points out that a chair can be a positive or negative symbol. It can represent a place where we experience comfort and rest, where we go to have a discussion with another or even ourselves—looking within. It is, then, said to be symbolic of receptivity. In other words, it puts us in a place where we can think, open our minds and our hearts; make decisions; or, solve problems.
The chair can also have a negative connotation: if it is a chair that has been treated poorly, or if we sit passively in it without recognition of the world around us, or any "discernment" (understanding) it can be a negative symbol.
Of course, the meaning of symbols are not written in stone. Some symbols are relatively common. We see a dove and many of us think of peace, or perhaps of Noah's ark when the dove returned with a twig in its beak. The sign holding up two fingers in the shape of a "V" is symbolic today of peace, whereas in generations past, it symbolized victory. Green can symbolize envy (jealousy) or money. So assigning meaning to the chair is a rational exercise, but one that is specific to what you perceive it to mean. You may find clues in the writer's repetitive use of the symbol (to recognize it as a symbol) and its meaning to you may be how you see it work in the author's writing.
Based on all this, I believe that the speaker will get past her pain, for she does venture out into the world. When she comes home, she arrives to loneliness, but she still goes out. Seeking out a time of rest makes me think that she will recover. Perhaps after a while she will stay out longer each time.
So for me, the overriding symbol is the chair, which she uses to represent her loss. Since a chair can symbolize rest, I think the speaker will spend some time resting in the moment; but one day, she will remove the chair from the house...she will let the past go.