I don't understand why her younger self didn't just tell Moonyoung her circumstances. Even if they don't know each other well, talking to him sincerely was her best shot. It's so easy: "Hey you have the letter I wrote. I don't want that letter found since it could ruin my friendship. Can you please destroy it, if you're not giving it back to me?" Simple. She coule elaborate more if he's curious how exactly that letter threatens her friendship, but that was step one.