...I forgot to mention, concerning why these particular plays and passages appealed to young females of the time, that Isabella Thorpe says something that is very like a thought from the very same act and scene of Twelfth Night as the passage Catherine memorized (Patience on a monument Act II, Scene IV)

"Everything is so insipid, so uninteresting, that does not relate to the beloved object! I can perfectly comprehend your feelings."

In Act II, Scene IV, Orsino says:
"For such as I am all true lovers are,
Unstaid and skittish in all motions else,
Save in the constant image of the creature
That is beloved."

The same act and scene also has the Clown (Feste?) singing this, which I think would be appealing to these girls who like gothic literature:

Not a flower, not a flower sweet
On my black coffin let there be strown;
Not a friend, not a friend greet
My poor corpse, where my bones shall be thrown:
A thousand thousand sighs to save,
Lay me, O, where
Sad true lover never find my grave,
To weep there!