The whole letter can be read here. My copy of this book is an dogeared mess and the three and a half pages that take up this letter and the initial query are what I come back to the most.
The overly personal as tied completely to universal truth, e.g.:
“One hot afternoon during the era in which you’ve gotten yourself ridiculously tangled up with heroin you will be riding the bus and thinking what a worthless piece of crap you are when a little girl will get on the bus holding the strings of two purple balloons. She’ll offer you one of the balloons, but you won’t take it because you believe you no longer have a right to such tiny beautiful things. You’re wrong. You do.”
There are several other paragraphs I would paste here, but imagine it’s worth just reading the entire thing. As advice, as a narrative, as a piece of writing, I continue to learn from this letter (and the rest of the collection).