I take it you are referring to the Tankas that are in the Holt Textbook for Grade 10 that come straight after this great poem by Dickinson. Remember, when we think of mood, we are referring to the emotional impact of the poem--the way it makes us feel when we read it. All of these poems deal with the attempt of the speaker to move on after a relationship that has ended for one reason or another.
In "Heart! We will forget him!" the poem starts off in a defiant way, addressing her heart with an order to forget the relationship. However, this defiance quickly moves into a mood of sadness as the speaker realises the things they need to forget:
You may forget the warmth he gave--
I will forget the light!
The speaker's command for her heart to make haste in forgetting the loved one and the nagging possibility at the end of the poem that the speaker will "remeber him" shows the deep mourning and sadness of the speaker and the tone matches this emotion.
Instead of sadness, the predominant mood of the three tanks seems to be bitterness and pain. Note how images such as "darkness," "drifting ship," "drenched / In cold waves" and "my life has emptied itself" create an incredibly barren and harsh image. In addition, the intensely personal nature of Tankas and the way they were delivered to a specific individual makes the anger and the bitterness more acute. In one, the Tanka laments the fact that the result of the relationship has been that the woman has become "like this stalk of grain" that has emptied itself. In another, the indecision of the lover is characterised by the "drifting ship" that only serves to "drench" the speaker in "cold waves."
Whereas "Heart! We Will forget him!" focuses on the positive aspects of the lover and creates a mood of sadness as the speaker tries to urge herself to move on, the Tankas are much more personal and pointed in their attack on the lover. The anger and bitterness and also the feeling of emptiness are self-evident, as created through the diction.