Before novelist Terry McMillan’s meteoric rise in the 90s, a publisher might have laughed at the suggestion that a quiet novel about an ambitious black couple falling in and out of love could sell a million copies. There might even have been the insistence that a “black novel” needed to be literary – a ripe exploration of perseverance in the style of Toni Morrison and Maya Angelou – and not a contemporary romance.

McMillan proved this mindset wrong with her 1989 novel Disappearing Acts, a New York Times bestseller. She proved it wrong again with Waiting to Exhale, in which four black women search for “Mr Right”. And again with how Stella Got Her Groove Back, a book about a divorced mother falling in love with a man half her age (“That kind of thing was taboo back then,” McMillan says). And then … well, you get the point.

However, McMillan, whom Publishers Weekly once called a “thwarter of book biz gatekeepers”, does not like looking back. “I’m just getting started,” says the 68-year-old, who now lives in Pasadena, California. “I don’t feel old or like I’ve done everything I’ve wanted to. I really don’t.”

Photograph: AP

This guiding principle of looking forward also runs through McMillan’s latest novel, It’s Not All Downhill From Here. The book, McMillan’s 10th, opens with Loretha Curry, the owner of two beauty supply stores, preparing for her 68th birthday. Loretha has had a bumpy life – two failed marriages under her belt, estranged from her fraternal twin sister and daughter, and multiple friends unexpectedly passing away before their time. But Loretha is determinedly happy and satisfied. Her third husband, Carl, whisks Loretha away for a surprise celebration in Palm Springs. But a sudden tragedy forces Loretha to build her life over from scratch and decide how she wants to live her final stage of life.

With Loretha’s coming-of-age at 68, McMillan strives to refute the idea that a black woman’s life loses relevance at a certain age.

“I wasn’t sure anyone would care about this story,” McMillan admits. She says the media interest in It’s Not All Downhill From Here is surprising. “It’s not that I don’t think it’s good. It’s just … this is a story about a 68-year-old woman. I wondered, ‘How many people are going to read this?’”

McMillan’s self-doubt comes even after she’s sold millions of books, co-written two successful film adaptations of her bestsellers, and sparked the “Terry McMillan Effect” – a phrase some use to describe the publishing industry discovering how starved for relatable stories black women were. In 1992, the paperback rights to McMillan’s Waiting to Exhale sold for a reported $2.64m, one of the highest prices paid for reprint rights at the time.

At the time, for women, dating a younger man felt like a dirty secret Terry McMillan

McMillan’s novels highlight complicated, successful black women of various ages and backgrounds. Her protagonists serve as predecessors to characters such as Olivia Pope of Shonda Rhimes’ Scandal, Cookie on Empire and the women of Tyler Perry films.

McMillan says she always strived to push boundaries with her plots, frequently pulled from her own life. How Stella Got Her Groove Back, released in 1998, focused on a woman falling for a man 20 years younger. It was inspired by the early days of her romance with Jonathan Plummer. “Nobody talked about it back then,” McMillan says of their initial romance. “At the time, for women, dating a younger man felt like a dirty secret.”

Breaking taboos proved to be a success for McMillan. The first printing of How Stella Got Her Groove Back was one million copies.

What advice does McMillan have for young black writers?

“Don’t try to write a bestseller,” she says sharply. “Every time I sit down, I just think about the characters. I don’t think about the movies or the money or any of that stuff. You can’t! You just have to write what feels honest.”

The publishing landscape was different in 1987, when McMillan’s first novel, Mama, was released. “The publisher wouldn’t send me out on a book tour,” McMillan reflects. “All the other white writers got one, but not the black ones.” So McMillan funded her own tour across the nation, visiting historically black colleges and universities and black-owned bookshops. Before long, McMillan’s name was a staple in black communities.

It was McMillan’s third book, Waiting to Exhale (1992), that saw the author become a true crossover success. The novel is a black predecessor to Sex and the City: it follows the romantic trials and tribulations of four close friends as they search for romance. It was turned into a box-office hit in 1995, starring Whitney Houston, Angela Bassett and Loretta Devine. Suddenly, the New York Times and New Yorker were profiling McMillan.

McMillan’s success opened the doors for many black writers. “I might have spent years trying to publish my black horror fiction if not for Terry McMillan,” the author Tananarive Due tells the Guardian. “After Terry’s success, black writers would often receive lavish book tours for their hardcover releases and then another book tour a year later for the paperback. One publisher gave me a company Amex card! This probably still happens now, but it’s not happening to the degree it was for a lot of us in the 1990s, when publishing was giddy with the discovery of black readers and was willing to take chances on a wide variety of writing to court that audience.”

But the fame and influence had its drawbacks for McMillan. Despite her large readership, McMillan’s work never received the same care and attention as writing by authors such as Alice Walker or Toni Morrison. Even though she has consistently written some of the most powerful, honest depictions of black womanhood, McMillan’s work is viewed as “popular black fiction”, which has associations with “smut” and “guilty pleasure” that have been hard to shake off.

“I never liked that label, ‘popular black fiction’,” McMillan tells me. “That just because something is popular that means it’s bad.”

In 2005, the focus shifted from McMillan’s writing to her personal life. She became the topic of blogs when her husband, the inspiration for How Stella Got Her Groove Back, came out as gay. She appeared with him on Oprah twice to discuss their high-drama affair.

A decade later, McMillan says she does not regret making her private heartbreak national TV fodder. “[Jonathan] and I went on Oprah because we were actually still friends,” she explains. “I don’t have regrets about doing that. I really was in love with him for the 10 years we spent together. I really was.”

McMillan says she never let the labels or public drama get under her skin. That she stayed focused on improving her writing and besting herself – even though she had already earned millions. “I never assumed that everything I have would be here the next day.”

I ask McMillan if she keeps tabs on the growing controversies and missteps in publishing. “Yes, I’m quite active on Twitter sometimes,” she says, with more than 260,000 followers. When asked specifically about the controversies surrounding hot-topic books, such as American Dirt and My Dark Vanessa, McMillan declines to answer at first. “I believe everyone should be able to write about what they want,” she answers, diplomatically.

I’ve never been interested in writing anything I don’t know about Terry McMillan

She pauses, then provides this important qualification: “But I’ve never been interested in writing anything I don’t know about. Like, The Help.” She’s perplexed why a book written by a white woman about segregation received such outsized attention in pop culture. “I could never write a book like that. But if I did, I’m sure it would be pretty successful and taken more seriously.”

McMillan continues exploring the topics relevant to her. It’s Not All Downhill From Here is noticeably different from her most popular works. Readers might be disappointed to find the pages aren’t brimming with fevered sex and romance. You won’t find a charming, attractive Jamaican man here. Instead, an unexpected loss sparks a series of reconciliations and personal changes for Loretha. McMillan, who typically spends a year on each manuscript, acknowledges the novel is a tonal shift. “I didn’t know Loretha was going to lose someone important to her when I started writing the story,” McMillan says. “I started crying when I wrote the [death] scene. I say, ‘Oh, they have to die. This is what has to happen.’”

McMillan uses the major death to illustrate that age should not be used as a measuring stick for what “should” happen in your life. We can come and go at any time. And also change.

“I don’t feel old, ” she says, “not at all.” She says she’s surprised by this feeling – when she was younger, 68 sounded like a death sentence – and she wants other black women to realize that age does not define how you live your life or when it will end. “When I went to my high school reunion a few years ago, over 90 people had already died. And we were not even 60!” she says. “Far too young for that many to be gone already. I was like, what is going on? It made me realize, you should always be grateful to be alive.”