When Chidera Eggerue, now 23, was a teenager, she knew exactly what she wanted for her 18th birthday. “I told my mum: ‘When I turn 18 and get a job, I’m getting a boob job,’” she says, laughing. “Seriously, what hypothetical job for an 18-year-old was ever going to pay for a boob job? They’re, like, eight grand. But I just thought: ‘I’m going to get a boob job and then I won’t be sad about my boobs any more.’”

The south Londoner never went through with the surgery. Instead, she had a revelation about body image. That she, a young woman, not even 18, felt her boobs were “too saggy” and not the perfectly rounded, pert shape that dominates mainstream images of women, was something that needed to be challenged. She decided to leave her boobs as nature intended: without interference – and without a bra.

“At the time, it was only small-boobed women who were ‘allowed’ to not wear a bra – because there wasn’t much to judge,” she says. As a “larger-boobed woman”, her decision attracted a lot of unwanted attention. “People would say: ‘You’re jiggling too much. I can see your nipples. This is bad. Cover yourself up.’ I couldn’t understand why. Every time I asked somebody, the answer was: ‘Because you’re a girl. Because you’re a woman.’ I knew that wasn’t really an acceptable answer. I had to challenge it.”

Shortly after that revelation, Eggerue started her fashion blog, the Slumflower, which aimed to provide an antidote to “mainly white and middle-class” fashion sites that propagated the idea of an unattainable and unrelatable lifestyle and look. “The content that they put out was aimed at people who looked and lived like them: Max Mara handbags, fedora hats and expensive camel coats.

“I couldn’t relate to them so I decided: I’m going to start a blog, and it’s going to be conversational, about fashion and bodies, but also just about feeling more confident, especially as a black girl.”

The Slumflower focuses on modern street style, showcasing affordable and vintage labels. Much of it features images of Eggerue, standing confident (and braless) in outfits she has styled, while written posts look at body issues, confidence and their interplay with race and gender issues. “I found my voice,” she says. “I decided I wasn’t going to wait for anyone to give me permission to use it and speak about my life as a black woman pursuing a creative career, having big hair and trying to maintain as much of my identity as possible in a world that tells you that you shouldn’t be yourself.”

Eggerue, posting on Instagram in June. Photograph: @theslumflower/Instagram

Her profile increased rapidly, and with it, so did a concept: Saggy Boobs Matter. A hashtag was born as women around the world joined in to post their braless pictures. A scroll through the hashtag reveals the diversity of women the #SaggyBoobsMatter message touched. Women of all ages, sizes, shapes and races could be found proudly posting pictures of their breasts that didn’t necessarily match the media image of the light-nippled, scar-free, symmetrical, perky breast. Meanwhile, Eggerue’s blog and social output was finding a larger and larger audience, including an appearance on ITV’s This Morning. She was becoming a millennial agony aunt, though women of all ages were reaching out to her: “I’ve had women who are 50 message me on Instagram to say they’re glad they discovered me and my Saggy Boobs Matter movement.”

As well as her viral posts on toxic relationships, or the #BlockHimParty hashtag – which encouraged women to celebrate their independence and block the phone number or social profiles of “emotionally unavailable” men – Eggerue offered tailored practical advice to the modern young woman through her message of big-sisterly “harsh truths” and self-love. Much of this has now been compiled in her debut book, What a Time to Be Alone. The book presents itself as a guide for women to be happy in themselves, and is split into three chapters: You (“celebrate yourself”); Them (“don’t worry about them”); and Us (“feel the togetherness”). It is presented using colourful graphics and features Igbo phrases and proverbs from Eggerue’s Nigerian mother. (“When the rat follows the lizard out into the rain, it is only the rat that gets soaked.”)

She says that, like the blog, the book aims to be an antidote to the mostly “first-world, privileged” self-help book scene. “As a black person, when I’ve read some of the self-help books that tell me: ‘Quit your job and travel,’ I’m like: ‘Really? I don’t think I can afford to do that …’ You get this feeling of not being good enough – not Zen enough. I didn’t want this to be one of those books. They’re fine, but they don’t equip you with immediate solutions.” Eggerue explains that there are no page numbers in the book, in order to emphasise that change is not a set journey with a beginning and an end.

Fans of Eggerue will know that she is no stranger to discussions about patriarchy, racialised standards of beauty or internalised misogyny. But overtly political lines of thinking are given barely any space in the book. While a message of self-determination outlining that any woman can be who she wants to be is a noble one, it can at times feel incomplete. This feels especially true in sections relating to anxiety and “letting go” of trauma. Yes, some women may be able to do this, but others may need professional help. In this context, lines such as, “Your victim mentality is why you are stagnant,” can be jarring and at odds with other sections of the book that recognise how the emotions of women are routinely minimised. It should be said that the existence of Eggerue as a popular figure in a largely white body-positive movement is a political triumph in itself. But the question remains: how can we have an earnest conversation about empowerment without discussing the social structures that disempower us?

When I ask Eggerue about this, she counters that the book isn’t a cure-all and, at least if it can help some people want to seek professional help, that is surely a good thing. I am inclined to agree – but I push on the point about simplistic answers to complex questions, or rather, apolitical answers to political problems.

Facebook Twitter Pinterest Appearing on This Morning to discuss #SaggyBoobsMatter. Photograph: Ken McKay/ITV/Rex/Shutterstock

“There just isn’t a one-size-fits-all answer,” she says. “It’s like the question: ‘How do we end racism?’ There just isn’t a way.” Eggerue admits that she can be a nihilist, which is also something that separates her from the saccharine world of self-help. When asked how she stays happy, she replies: “Does anyone stay happy? I think happiness is fleeting and I’m learning to accept that. I think becoming a nihilist is something that happens when you have been disappointed so much that you realise that everything is temporary; happiness is fleeting.

“That’s why I feel there has to be a way for someone to find value in themselves, where they say: ‘I do deserve to be here. Maybe society doesn’t view me as valuable, but there is still value in me.’”

Anyway, she says, there is a value in not sliding into political language in the book. “Terms like ‘patriarchy’, or ‘historical racism’ – it’s easy for the jargon to become your normal conversation, but if I want to introduce people to that conversation, I have to start from where they are. So as much as I really did want to dig in to gender politics and understand how it all works, it’s important that I also make it accessible. It’s why I use the Igbo proverbs. Everyone can relate to animals.”

Did she ever feel frustrated by her commitment to accessibility? “I actually preferred scaling back. It was relieving to not have to vent about capitalism or internalised misogyny, not least because one of my biggest challenges has been winning my mother over in terms of getting her to stop fighting with me about feminism. When I’m having conversations with her, I can’t be using terms like ‘internalised misogyny’. I have to break it down for her because when I use words like that she closes off and becomes intimidated, and thinks I’m trying to be a smart-mouth. And so she stops listening to me.

“One thing I’ve learned is that when you’re trying to correct someone on something that they’ve believed their whole life, you have to be gentle with them because you’re deconstructing their reality. But if you use a more accessible term, then people are more likely to be receptive to the message. For me, my mum represents the world.” Which raises the question: has she yet persuaded her mum, and is the next stop the world? Not yet, she says, laughing. “But I’m going to try.”

What a Time to be Alone by Chidera Eggerue is published by Quadrille (£12.99). To order a copy for £11.04 go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99