The last sentence is very formal. It does not contain the first person pronoun (I or me) which is one of the marks of informal writing. Having given you the answer, may I say that question is almost totally irrelevant. I do not mean that you are at fault for having asked it. You are only asking for an answer, and that was given.
What I really mean to say is that I hope you get to read more of Amy Tan's remarks about her mother's English. They are astonishing observations and should be required reading by every student taking English or American Literature.
Thank you for posting this question. I didn't there was more to Tan's essay about her mother's English that I didn't know about, and I'm glad to know there is more.
The image of "a huge mountain 'tween my heart and tongue" suggests that Portia does not want to say what she is feeling.
When Lucius asks Portia why she sends him to the Senate, she hesitates to answer him; her mind is put elsewhere. Portia wants to be strong enough not to reveal what she knows about Brutus' plans, since he considers Caesar has to be killed for power abuse. She cannot keep such secret anymore, and begs "constancy" to help her maintain composure.
Audience analysis is the term usually used.