Catherine describes her secret. That is to say, she tells Nelly that she has received a request of marriage from Edgar Linton and also that she has given him an answer. Her dilemma is that she does not know if her answer was correct and is asking for Nelly's guidance. This shows Catherine's underlying insecurities and her association with Nelly and wisdom.
Catherine continues, and finally says that she has accepted Edgar's request. For Nelly to give an answer to whether she thought this was appropriate, she asks some questions that reveal more about Catherine's character.
Catherine's answer to the question "Why do you love him?" shows her naivety about the subject of love. She answers with attributes of Edgar that would change over time, and may fade altogether.
Her only 'adult' answer was that Edgar loved her back, which shows the strange lack of experience that is only possible to attribute to the isolation of the Moors.
These superficial clichés described, Nelly proceeds to show Catherine that the reasons she has given for loving him are comparatively thin when compared to other men who have more material wealth etc.
Once Catherine exclaims that she has "only to do with the present", she confirms her own childishness and ignorance.
Nelly listens to Catherine describing how, although, on the outside the marriage looks without problems, Catherine feels that there is something wrong in her soul that she can not decide the cause of.
To explain this to Nelly she compares how, because she had deemed herself a sinner; she has "no more business to marry Edgar Linton" than she has to be in heaven.
Catherine wants to be 'higher class' than she is now, and live with a better lifestyle that she would achieve if she married Edgar. She said, "it would degrade me to marry Heathcliff", and that "he shall never know how I love him; and that, not because he's handsome, but because he's more myself than I am". This sentence describes how Catherine believes that she would lead a poor life if she married Heathcliff, but is saddened by the fact that because of this, Heathcliff will not know her feelings towards him.
Catherine knew that her and Heathcliff's souls were 'made' the same, whereas hers and Edgar's were as "different as a moonbeam from lightning, or frost from fire".
She though that their souls were not as compatible as she first though, showing again her tendency to rush into things and act in a rash, childlike fashion.
Catherine realises – with Nelly's help – that by becoming Edgar's wife, Heathcliff will lose a close companion and friend and tries to make herself believe that she was right in doing so.
She then counters her argument with the frankly unethical idea that she will use Edgar's money to support Heathcliff, although she knows that this is entirely the wrong reason for marriage.
Catherine loves Edgar, not for what he is or who he is, but for what he can offer Catherine and how she can use it to her advantage. Her bond with Heathcliff is much stronger, and goes beyond possessions and wealth. She believes that on a higher level, she and Heathcliff are the same, and that they are impossible to separate.
At the same time she is showing her inability to make decisions and also a lack of moral code that most people would find acceptable, for instance she intends to 'use' Edgar as a source of help for Heathcliff. This could be interpreted as correct, but in essence it is wrong.
I can see this set of circumstances ending in tragedy.
