How has the speaker evolved from her former “bitch” self?

The failure of the speaker’s ex-lover to reciprocate with love and affection, and his “ultimate dismissal” of her are the factors that cause the speaker to evolve from her former “bitch” self. The speaker, however, never reflects her "bitch" self while she converses with her ex-lover. She refrains herself from breaking down and is in full control of herself.

Her inner self, meanwhile, recalls the period when the couple were together. She remembers,

“…how she...

The failure of the speaker’s ex-lover to reciprocate with love and affection, and his “ultimate dismissal” of her are the factors that cause the speaker to evolve from her former “bitch” self. The speaker, however, never reflects her "bitch" self while she converses with her ex-lover. She refrains herself from breaking down and is in full control of herself.

Her inner self, meanwhile, recalls the period when the couple were together. She remembers,

“…how she came running   
Each evening, when she heard his step;
How she lay at his feet and looked up adoringly

This tells us that before the transformation in the speaker, she had been quite devoted to her lover. She was submissive and loyal to him and seemed to be always ready to do anything to please him.

But her ex-lover didn't seem to be as fond of her as she was of him. Instead of returning her love, “he was absorbed in his paper;” “Her devotion” bored him, and he would order “her to the kitchen.”

He would love her infrequently, only “when he’d had a good day, or a couple of drinks.”  He returned her love with his “casual cruelties.” Finally, he dumps her.

It had hurt her deeply. But she had to move on, and she has. She tries to console her inner "bitch" self saying, 

"He couldn’t have taken you with him;
You were too demonstrative, too clumsy,
Not like the well-groomed pets of his new friends."   

Now, this explanation offers a deep psychological insight. 

It’s possible that she’s not really sure about what the reason could have been for his dismissing her. She has always known herself to be highly devoted to him and very caring.

So, she tries to explain her inner self in a self-deprecatory way:

You were too demonstrative, too clumsy,
Not like the well-groomed pets of his new friends.

In this way, with time and with lots of pain, she has evolved herself from her inner "bitch" self. Though she has evolved and never breaks down emotionally before her ex-lover, she can't help being drawn towards him.