Gabby Bernstein is exhausted. It's not only jetlag that's killing this blonde, skinny New Yorker during her trip to London: she's just given an intense two-hour talk to hundreds of devotees who have paid more than £100 for a ticket. The striking 35-year-old looks like a Jewish princess - ripped skinny jeans, straight hair and stilettos - yet is anything but. She's a successful New Age teacher: in her own words, a "spiritual lightworker" capturing a new generation of happiness-seekers.

"I'm sorry I'm so tired," she apologises, as she orders a pizza that she slumps over in increasing gradients during our chat. I assure her that it's OK, having just watched her give everything on stage: utterly centred and passionately holding court for 120 minutes. She inhabits her words: she laughs and cries and swears liberally as she gives advice, tells stories from her life and answers audience questions. If you had no truck with her message, you would at least be impressed by her stagecraft.

She's forthright, articulate and clearly driven by her mission to spread her message. That message, broadly, is this: you can be happy, and you must spread your happiness into the world. Those are the goals, and here's the method: meditation, forgiveness and prayer. It's all served up in Bernstein's no-nonsense language. "Say thank you for your s***," she tells followers. "It brings you to where you are and allows you to change." This is a woman who high-fives me when I tell her about a career bump, promising it's happened to make way for a better future.

If the message sounds fairytale-chirpy, the story behind it provides a harsh context. Aged 25, Bernstein broke down after years of multiple addictions (relationships, work, drink and drugs). She begins her talk with a quote from a spiritual text that set her on the road to recovery: "There is a way of living in the world… you smile more frequently. Your forehead is serene; your eyes are quiet." She soon realised that she wanted to teach others how to achieve that serenity. She gave her first talk four months after becoming sober. Now she's an author who flies around the world speaking, coaching and posting motivational messages to her 125,000 Twitter followers and 165,000 Facebook fans.

Bernstein is quick to defend the notion of spirituality: "Spirituality isn't a luxury. It's a must. That goes for any kind of faith - Jewish or spiritual or however you find it. Without faith or spirituality, you think that this is up to you and that you're navigating everything and that there's nothing that you can lean on. But when you start to trust that there's something you can lean on, that's when things get really beautiful."

Bernstein's not religious but her Judaism is still evident. She enjoys regular, "very spiritual" Shabbat dinners with friends. ''I think religion is a gateway for spiritual awakening."

One example is her wedding to husband Zach Rocklin, who recently quit his banking career to become his wife's business partner. "My best friend, who's my most religious friend, married us in a standard Jewish format but there was no rabbi. We're not religious so what was most authentic to us was a spiritual wedding." Rocklin is the person who delivers Bernstein's pizza; giving her an affectionate squeeze as he does so, he comes across as a calm counterpoint to her bold personality.

Bernstein's traditional Judaism was stronger in her youth. "I was very into the religion growing up. I went to Jewish summer camp. I led a youth group, which was very similar to the work I do today - I was leading groups of young people in spiritual conversation. It definitely prepared me for the path I'm on today."

Bernstein's audience remains young. I chat to a stream of fans - mainly women in their 20s and 30s - who all praise their mentor's "relatability". Some place sufficient trust in her teachings to make huge life changes. Sam, 31, has flown from Belfast to London to hear Bernstein. She left her job and went freelance after reading the bestselling Miracles Now: "I'm living an authentic life because of Gabby. Maybe it's the way it's all packaged. She wears nice clothes. Maybe I'm like her."

During the talk, Sam asks Bernstein whether she should leave her partner. Bernstein steers clear of giving her a yes or no. Does she feel responsible for her followers? "No, I don't feel responsible for other people. If I did I wouldn't be alive. I'd be sick on the floor. My job is to wake people up and then it's the individual's job to carry on. I feel like the biggest responsibility is to practise what I preach and that's quite helpful because it keeps me in my practice and committed to my path."

Bernstein's new book is The Universe Has Your Back. Its premise is that the universe ("or higher power, or God, or whatever you want to call it" says Bernstein) can help you if you call on it. But what about people suffering under ISIS, or gang-rape victims in India, I ask?

"Well, miracles are available to all of us but those of us who have the privilege of being in a country that is free and a safe environment have a responsibility to live at our highest capacity, bringing a higher energy to the world so that we can support people around the world who don't have that privilege." But how does that work? "Own your joy so that you can be happier and bring more energy to the world. That ripple effect works on a quantum level. It brings about changes in the world."

I suggest this has parallels with kabbalah, which is underpinned by the notions of light and darkness and collective responsibility. Bernstein nods. "It's definitely aligned to collecting the light."

Tiredness finally takes over and our interview ends. Bernstein gives me a goodbye hug. I look at her face and her forehead is serene - no lines - and she's smiling. Some of her words might be esoteric but the physical evidence is clear. Gabby Bernstein got her wish.