While it seems possible to interpret the speaker as resolute and defiant in the face of frightening scenes and possibilities, I think the speaker is more likely powerless and unable to move. After all, she is "spellbound," or so we can assume from the poem's title: this means that she is held by or held as if by a spell, something she cannot control. Notice that at the end of each stanza, she says she "cannot, cannot go" and not that she will not, will not go. To say that she cannot go suggests that she is powerless to go, that she is unable to leave because of the "tyrant spell" that "has bound" her. If she had said she will not go, then a choice is implied. Therefore, despite the frightening scene developing around her, she is unable to get away. She is forced to remain, in thrall and in fear, because she is "spellbound" and captivated by something outside of her control.

The speaker in this poem is alone, somewhere outside in a wooded area on a stormy, windy, snowy night, but for some reason the speaker is unable or unwilling to leave. "A tyrant spell" is holding him/her and preventing his/her departure. As the night grows darker and the winds get colder, the speaker "cannot, cannot go" - repetition that lends emphasis to the determination to remain in spite of the conditions.  Cold, dark, night - all are frequently symbols of death; the speaker seems to be in a perilous place, but something is holding onto him/her and not allowing death to come and take him/her.

In the second stanza, "the storm is fast descending" and is affecting things, and perhaps people, around the speaker, as "giant trees are bending, their bare boughs weighed with snow." Still, the speaker "cannot go."

In the third stanza, the speaker has visions of "Clouds beyond clouds above me" - heaven, perhaps - and "Wastes beyond wastes below" - the depths of hell. There is a power or wish to live that is stronger for the speaker than these powerful images, however.
 "But nothing drear can move me, I will not, cannot go." The addition of "I will not" emphasizes the speaker's determination to remain, not allowing all the perils and troubles to conquer his/her spirit.