When I was 15, I thought I knew heartbreak. I had just gone through my very first breakup — two whole months of true love down the drain — and I was devastated. Or at least, I thought I was. In reality, it was probably Diane Warren who broke my heart. Every single day, I would sit on my bedroom floor, clutching my pink iPod Shuffle, wailing along to LeAnn Rimes’ “How Do I Live.”

“How do I live without you? I want to know

How do I breathe without you? If you ever go…”

Did I need to ask these questions about a guy who got me a box of Oreos for my Christmas? Absolutely not. But Warren’s words were so dramatic, so gut-wrenching, that I felt a heartbreak even if I hadn’t earned it yet. That’s what Warren does best: forces you to feel something, whether you want to or not.

The documentary, “Diane Warren: Relentless,” which screens this weekend at the Boulder International Film Festival, finally pulls back the curtain on the woman responsible for some of America’s most emotional, tear-stained anthems. Songwriters rarely get their moment in the spotlight — listeners tend to credit the singers, forgetting there’s an invisible hand behind the lyrics we scream into the void (or the back of the girls bathroom during third period). But Warren has written so many iconic songs (“If I Could Turn Back Time,” “Because You Loved Me,” “Unbreak My Heart,” “I Don’t Want to Miss a Thing”), that her impact is undeniable, even if most people wouldn’t recognize her on the street.

The film does a fantastic job of capturing Warren’s contradictions: her songs are achingly romantic, but she herself seems entirely uninterested in love. (Talking about her hit song, “I Don’t Want to Close My Eyes sung by Aerosmith for the movie “Armegeddon,” Warren says, “I don’t want anyone staying up all night kissing my eyes. Ew. What are you kissing my eyes for?” she scoffs.)Her melodies are polished to perfection, but her house? A disaster — the documentary opens with a slow pan through her home, a chaotic mess that feels more like the bedroom of a teenage girl with a songwriting obsession than the residence of a multi-award-winning legend.

Then there’s the matter of Diane Warren herself, who is — as Cher succinctly puts it in the opening scene — “nuts.”

Cher and Warren go way back. The two have worked together for decades, though not always smoothly. In one of the film’s funniest anecdotes, Warren recalls writing “If I Could Turn Back Time” for Cher, only for the star to hate it with a burning passion. “I literally got on my knees and begged her to record it,” Warren says. “I told her, ‘I’ll pay for it. If you don’t like it, don’t use it.’” Cher, with all the reluctant exasperation of a mother indulging a persistent child, finally gave in. And as soon as she started singing it in the studio, she turned to Warren with a glare that said: “You b****, you were right.”

This moment is one of many that showcase Warren’s relentless nature. In the film, she’s eccentric, neurotic, and by her own admission, “a pain in the ass.” She refuses to take no for an answer when she believes in a song, writing and rewriting it until she achieves her very strict idea of perfection. As a young girl, her parents would often lock her in the backyard shed because she would play songs over and over and over until they were just right. As a writer, doesn’t believe in inspiration or a strict creative process — instead, she just sits at the piano and plays until the music comes out.

The documentary is mostly a straightforward talking-head format, with interviews from artists such as Jennifer Hudson, Lady Gaga, music execs, as well as her close friends and family, and all trying to sum up what makes Warren, well, Warren. The pacing is lively, mirroring her restless energy, with a mix of archival footage, behind-the-scenes clips, and a truly hilarious moment where we hear Warren herself singing the original demo for “If I Could Turn Back Time” — Let’s just say it’s a good thing Cher took over. But “Relentless” isn’t a total lovefest. The film acknowledges that Warren is, at times, a difficult person. She famously gave “How Do I Live” to both LeAnn Rimes and Trisha Yearwood at the same time, sparking controversy in the industry. She’s obsessive, blunt, and, as music mogul Clive Davis says in the documentary, has “never been in love” — a fact that makes her ability to write devastating love songs all the more fascinating. Warren doesn’t write from personal experience; she writes like a method actor, embodying the emotions of the people who will eventually sing her words.

Her personal life remains largely private, but we do meet Mouse, her cat, and her bird, named Buttwings, who might be the only beings she truly loves. “She has a lot of friends,” a close friend of Warren tells the camera, “but she’ll tell you that she prefers animals over people.”

There’s a quote from “Relentless” that sums up Warren’s essence perfectly. Randy Jackson, in describing her, says: “The combination of things that she is is so bizarre: she’s really awkward, she’s really shy, she doesn’t like to speak in public…This is the same girl who’s teaching her bird to say ‘(expletive) you’ while writing ‘Unbreak My Heart.’”

That’s Diane Warren in a nutshell. A woman who is both incredibly tender and utterly unsentimental. Who is wildly eccentric but methodically precise. Who, whether or not we realize it, has made us all cry at least once in our lives.

And if you’re anything like me, it was probably when you were 15, sitting on your bedroom floor, iPod Shuffle in hand, wondering how you’d ever survive.