“Once upon a time,” on an afternoon in 1991, my family gathered around the television. We had been alerted by my sister Blazey to the airing of American Playhouse’s recording of Into the Woods, a musical by someone called Stephen Sondheim.

Across those 2 hours and 31 minutes, I experienced something akin to a paradigm shift: a musical, it was now clear to me, could be more than just pretty melodies and glittering costumes. It could be erudite, scary, thrilling, as funny as Blackadder and as moving as National Velvet (such are the high watermarks of the nine-year-old’s pantheon).

But there was something deeper at play for that (then-undiagnosed) autistic child: in the emotional ambivalence and lyrical dazzlement of songs like Moments in the Woods and On the Steps of the Palace, I felt, somehow, recognised. I quickly set about consuming every skerrick of Sondheim’s work that I could get my hands on.

In Sondheim’s music and lyrics I had found a trusted friend and confidante, a harmonist to harmonise with. There was something irresistible in the rhythms and dazzling word puzzles of his lyrics that set synapses alight in my brain. This sad weekend past, I have been delighted anew by A Little Night Music’s Later:

“Though I’ve been born, I’ve never been!
How can I wait around for later?
I’ll be ninety on my deathbed
And the late, or, rather, later, Henrik Egerman.”

To paraphrase Jane Austen, what are Melrose Place and Smash Hits compilations to The Ladies Who Lunch and A Little Priest? This position did not brook me much schoolyard cachet; I once attempted to introduce Assassins to some cooler, older kids by shrugging, “It has swearing in it”. Learning Sondheim’s catalogue by heart was at first an act of musical perseveration, but his lyrics soon became my echolalic tenets. “He was,” as Mandy Patinkin offered this weekend, “simply one of our greatest teachers.”

Though my 11-year-old attempt to sing Company’s Being Alive at the school talent quest was squashed in favour of the more age-appropriate Getting to Know You from The King and I, Sondheim remained a daily part of my life for the ensuing three decades. I suspect it would have embarrassed him, even as he delighted in quiz shows, to hear that I once went on The Einstein Factor with his life and work as my topic. (And, in a positively Sondheimian result, came second to a Sherlock Holmes expert; Mark Humphries fared better on Mastermind.)

Reading, for what must be the 47th time, biographer Martin Gottfried’s Sondheim, I am drawn once again to his assessment of Sondheim’s character. “He seems afraid only of the cliched, the banal, the careless, and the unintelligent,” Gottfried wrote of Sondheim. “He is in this way a hero.” I lived my life in fear of the same things, not always successfully in the case of my own work, as fealty to his mighty influence upon me.

Having been introduced to his work at such a formative age led to bafflement at Sondheim’s undue reputation as a tetchy, un-hummable show-off. At 23, I brought Another Hundred People to a “BYO sheet music” sing-along at The Arts Centre’s Cafe Vic; I was somewhere around “By the rusty fountains and the dusty trees with the battered barks” when the accompanist gave up in a huff, and moved on to more crowd-pleasing fare. Later, I wandered back to my tram in a daze, and cried the whole way home. There is a breathtaking moment in Greta Gerwig’s Lady Bird, when Father Leviatch says of the school’s coolly received performance of Merrily We Roll Along, to nobody in particular, “They didn’t understand it.”

Burnt by the letdown of receiving a Xeroxed autograph from the office of Steven Spielberg at the tender age of 12, I never wrote to Sondheim to tell him what his work meant to me. That fear that he’d be terse in reply, or never respond, was eventually replaced by a sort of stage fright: how could I find the words to explain his immense influence upon me? (As countless stories shared in the aftermath of his death have demonstrated to the contrary, he was in fact a generous and committed letter-replier.) Just a week ago, unpacking my Sondheim books as I moved house, the thought once again rose: I really need to write to him, and soon.

There was a part of me that genuinely believed with trademark autistic magical thinking, when I first travelled to New York at the not-so-tender age of 28, that I might just bump into him somewhere and be able to tell him in person that nobody – no teacher, mentor, friend or enemy – has made a bigger impact on my life than Stephen Sondheim.

It didn’t happen, so I’ll tell him now, and hope that, somewhere, he hears it. “I wish …”