In Emily Brontë's Wuthering Heights, what is the difference between the Catherine that loves Heathcliff and the one that marries Edgar?

In Emily Brontë's Wuthering Heights,
Heathcliff is "adopted" when Mr. Earnshaw (Catherine and Hindley's father) finds the boy
starving on the streets of Liverpool. Earnshaw and Heathcliff get along well, and
Catherine is very fond of him, too. The two youngsters spend a great deal of time
together growing up: while some people are frightened of the moors, Catherine and
Heathcliff find comfort there, each enjoying the company of the
other.

Hindley, though, is extremely jealous of Heathcliff
and the attention he receives, and he is nasty. Hindley beats Heathcliff as a youngster
and then Heathcliff holds it over Hindley's head, threatening to tell his father. It is
an unhealthy relationship. Hindley goes away to school, and when Mr. Earnshaw dies,
Hindley returns with a wife, and ownership of the Heights. From that moment on he makes
Heathcliff's life miserable.

Early on, until Catherine is
about twelve, she and Heathcliff have been devoted to each other. But Hindley's hand
in—and control of— Heathcliff's life takes its
toll.

Hindley's wife, at first thrilled with the Heights
and Catherine, soon becomes peevish. She develops a strong dislike
for Heathcliff. This is all it takes for Hindley to resume his torment Heathcliff. He
makes Heathcliff work with the laborers, bans him from associating as one of the family,
and he is no longer taught by the curate. When the Catherine and Heathcliff sneak off
together, upon their return she is punished with extra lessons, while Heathcliff is
whipped by Hindley. Every time they are caught, Heathcliff is
beaten.

One afternoon, Catherine and Heathcliff go to spy
through the windows at the neighboring home, Thrushcross Grange. While spying, the
Earnshaw "children" think the Linton children are foolish and ridiculous in their
behavior. Catherine is bitten by their dog. Though the Lintons are concerned for
Catherine, they are very rude to Heathcliff. He leaves, but Catherine must stay while
she recovers—for five weeks—and she grows to be friends with Edgar and Isabella. When
finally able to come home, she is a very different young woman: she has manners, has
been dressed in beautiful clothes, and been complimented excessively. She is more like
the Lintons and much less like Heathcliff.

This is when the
first Catherine disappears and the second one, who eventually marries Edgar Linton,
appears. She has no time for Heathcliff. His rough manners and behavior embarrass her.
While Hindley continues to abuse Heathcliff, he now has no support from
Catherine—he begins to change as
well.

Time passes and it seems that Catherine will marry
Edgar. While she speaks to Nellie, the housekeeper about this, she states that Hindley
has made it impossible for her to marry Heathcliff because of who he has become. Hearing
only this part of the conversation, Heathcliff sneaks quietly away and leaves the
Heights. What he does not hear is Catherine's confession, again,
that she loves Heathcliff very much.

With Heathcliff gone,
Catherine and Edgar marry—not that she is deeply in love with him. When Heathcliff
returns, he is educated, and a man of means (money). His childhood friend has changed
into a respectable young wife and their relationship is altered
forever.

Heathcliff is visiting with Catherine one day when
she is pregnant with her first child. She loses consciousness, gives birth and dies.
Heathcliff is devastated, and damaged beyond repair as a person. What he
might have been, he now will never
be.