How do Della's qualities help her resolve her conflict?

Della's conflict is an internal one; she desperately wants to purchase a nice Christmas gift for her husband, "Something fine and rare and sterling—something just a little bit near to being worthy of the honor of being owned by Jim." The problem? All she's been able to save is one dollar and eighty seven cents. After she cries for a while about the unfairness of it all, especially considering how difficult it is to live on so little as $20 a week, she seems to have an epiphany.

Suddenly she whirled from the window and stood before the glass, her eyes were shining brilliantly, but her face had lost its color within twenty seconds. Rapidly she pulled down her hair and let it fall to its full length.

Della is so giving that she does not seem to think twice before giving up her most valued possession so that she can purchase something beautiful for Jim. She is so selfless that it doesn't occur to her to just spend her $1.87 and be done with it. She is so joyful and loving and willing to sacrifice that, decision made, she has a "brilliant sparkle" in her eye as she "fluttered out the door" on her way to sell her most prized possession. She is also so mature and insightful that she realizes her love for Jim is more important than any material possession.

First of all, we need to establish the precise nature of the conflict that Della must resolve. As it turns out, it's the exact same conflict encountered by Jim. Husband and wife both want to express their love for each other by giving the very best Christmas gifts they could imagine. Unfortunately, Jim and Della are very poor, and therein lies the source of the conflict they both must somehow resolve.
For her part, Della attempts to resolve the conflict by selling some of her beautiful locks of hair to a hairdresser. Della doesn't have much in life, but one thing she does have is incredibly beautiful hair. By selling some of it to pay for Jim's Christmas gift, Della is showing not just her love, but also her selflessness. She is prepared to make a sacrifice in order to bring her husband a touch of happiness on Christmas Day. It's this extraordinary quality, more than any other, that helps her to resolve a seemingly intractable conflict.