First off we have the same name.
But I will answer your question if you answer mine. Do you mean to write a letter for teens to read (hypothetically)? As in " Dear teens, You need to take a look in the mirror and note all of the good things about yourself......"?
This is a great way to start but needs to changed you claim
If you're asking for main idea, the main idea or thesis statement sentence is sentence 1, where they explain what the passage is about.
Answer:
They Both Die at the End is a devastating yet uplifting story about two people whose lives change over the course of one unforgettable day. According to the book synopsis: “On September 5, a little after midnight, Death-Cast calls Mateo Torrez and Rufus Emeterio to give them some bad news: They're going to die today.
Answer:

<em><u>nice</u></em><em><u>❤</u></em>
<em><u>Never</u></em><em><u> </u></em><em><u>regret</u></em><em><u> </u></em><em><u>what</u></em><em><u> </u></em><em><u>makes</u></em><em><u> </u></em><em><u>you</u></em><em><u> </u></em><em><u>happy</u></em><em><u>¡</u></em><em><u>!</u></em><em><u>!</u></em><em><u> </u></em>