How do Mrs. Mallard's feelings change while she is in her room? Why might she fear this change at first but later welcome it?

In Kate Chopin's short story "The Story of an Hour," Mrs. Mallard, a woman described as having "heart trouble," learns of her husband's sudden death in a train accident. She weeps, then retires to her room to be alone. While she is in the room, she looks out the window and begins to notice all of the blooming life outside her home. She starts to think about the positive side of her husband's death: she is now free to live her life as she sees fit.
As Mrs. Mallard sits in her room, the following behavior is described by the narrator:

She was young, with a fair, calm face, whose lines bespoke repression and even a certain strength. But now there was a dull stare in her eyes, whose gaze was fixed away off yonder on one of those patches of blue sky. It was not a glance of reflection, but rather indicated a suspension of intelligent thought.
There was something coming to her and she was waiting for it, fearfully. What was it? She did not know; it was too subtle and elusive to name. But she felt it, creeping out of the sky, reaching toward her through the sounds, the scents, the color that filled the air.

Mrs. Mallard, at first, is not consciously thinking ("a suspension of intelligent thought"), but gradually, a feeling comes over her, without her control. This feeling, now "too subtle and elusive to name" morphs into her realization that she is now free. The narrator conveys,

She said it over and over under the breath: "free, free, free!" The vacant stare and the look of terror that had followed it went from her eyes. They stayed keen and bright. Her pulses beat fast, and the coursing blood warmed and relaxed every inch of her body.

This paragraph illustrates perfectly how Mrs. Mallard is at first afraid of her feeling ("the look of terror") but also how she quickly gets over it. Her eyes "stayed keen and bright." The description indicates excitement, which then leads to relief.
Mrs. Mallard may fear this change because the feeling is so new to her. As a wife in the 1800s, she is expected to dedicate her life to her husband, to raising a family, and to taking care of the home. She may not have realized until now that if she were not expected to prioritize those things over her own happiness, she may feel more free and more like an individual. She soon begins to look forward to her future, where she sees "a long procession of years to come that would belong to her absolutely." Unfortunately, and ironically, Mrs. Mallard's happiness is short-lived, as her husband is not actually dead and enters the home as she exits her room. When she sees him, Mrs. Mallard dies of a heart attack. The others in the home say she died of "the joy that kills," but readers know she was shocked and disappointed that her vision of her future would now not come true.