'Pretend that you're squeezing something between your legs," yelled the instructor over Sinitta's So Macho. "Neutral bottom, you need a neutral bottom. And … plié."

The coarse, hacking sound of fabric tearing was audible, even above the music. Then the laughter started. Less than three minutes into my dancing career, I was already a laughing stock. The instructor glided serenely across the floor. "Do you want a minute to grab a new pair of shorts?" he asked, failing to mask his amusement.

Before today, ballet was no more than a teenage chore to me. On Tuesdays, I'd collect my sisters from their lesson on my way home from school. Then I found it by turns funny (pink tutus and semaphore elbows) and mind-numbingly boring (a middle-aged woman playing piano and parroting "good toes, naughty toes"). Two decades later, here I was on a French dancefloor with a gaping hole in my shorts and an alice band in my hair. How had it come to this?

It had come via a programme of seven-night breaks called "in:spa retreats", run by the Healthy Holiday Company; it claims to reboot your body and mind via a mix of yoga, healthy eating and fitness. This week was its first foray into ballet.

I'd arrived at Marseilles airport, injured, out of shape and emotionally shellshocked after the end of a nine-year relationship. More of an ugly duckling with a broken wing than a black swan. Or, more accurately, a fat goose with a broken heart. After the toughest year of my life, I was here to rediscover some semblance of fitness and, above all, to dance away the blues.

Perhaps appropriately, the setting for my hoped-for renaissance was a grand pile in deepest Provence. Château de Robernier sits among vineyards on the outskirts of charming Cotignac – grand iron gates, turrets scratching at a cloudless cerulean sky and carefully clipped gardens giving a fairytale first impression

On arrival, we met the nutritionist, massage therapist and personal trainers for the week. The vast majority of the 16 guests were fellow Brits, with pretty similar goals. They were predominantly women in their 30s and 40s, and a scarily large percentage were looking to regain a sense of self after a crisis. Preferably a leaner, trimmer self.

The spine running through the week's activities was the barre itself. Long used as a training aid by ballet students, the barre has recently evolved into a broader fitness trend, largely in the wake of the 2010 Hollywood thriller Black Swan.

"We're not making you professional ballet dancers," says Malcolm Coombes, our "barre man" for the week – a former professional who now runs a dance studio in west London and counts Boy George among his clients. "Instead, we're approaching the barre and some simple ballet moves from a fitness perspective. It's high-burn, high-intensity exercise to high-tempo music."

A few seconds later – on my first plié no less – my "nutcracker" moment arrived. But after swapping my violated shorts for a pair of running tights, I got into the swing of things.

"If it's burning, it's firming," barked Malcolm as we each grasped the barre with one hand and tried to hold our pliés – posh squats in gym money – for as long as possible. After that, we went on to the fondu (the evil, one-legged brother of the plié), the battement tendu (extension of one leg) and the ronde de jambe (half-circles with a pointed foot). Throughout, we were told to imagine we were sandwiched between two panes of glass – "one in front of your chest and one behind your back" – to help with posture.

Then Malcolm put the whole thing to a remix of Michael Jackson's Billie Jean and suddenly we were dancing. That first session must have looked hilarious – flailing limbs all over the place, like a flash mob on an escalator – but it was so much fun. Laughter filled the gaping holes in our technique.

Each day began with yoga, followed by a healthy breakfast on the castle's sun-baked terrace. The strictly macrobiotic diet may sound rather terrifying but it was delicious and ideal for increased energy levels – and decreased weight levels. After breakfast came physical exercise, from a short run through the countryside to circuit training on the front lawn. Think Pride and Prejudice meets Rocky. With Dirty Dancing in the afternoon. Malcolm was always our instructor for the latter, but there was also a yoga teacher, Saskia, and another trainer, Kang, to keep us on our toes.

The barre sessions got smoother and more enjoyable as we became familiar with the basic positions. Soon we were moving through them at speed, with Robin Thicke and Flo Rida also regulars on our journey. Our studio was the chateau's dining room, so we were dancing beneath chandeliers and into and out of pools of sunlight split by grand, billowing curtains.

Barre fitness involved simple bodyweight exercises such as press-ups and sit-ups as well as ballet moves, so every two songs we would drop and hit the dancefloor in an entirely different fashion, while Malcolm floated up and down like a medieval ghost on steroids – albeit one familiar with classic denim: "We're going for the Levi's bum here. Can you feel it in your 501 region?"

It would be hard to better this setting for a restorative retreat. Whether it was the excellent diet combined with gluttonous exercise, or simply the sunshine and peace, the chateau seemed to hum with positive energy. It's only 40 miles from the noisy excess of St Tropez but it felt like a fortress of calm: the perfect place to reboot away from prying eyes. Every evening we turned right at the suit of armour, mounted the stairs and collapsed gratefully into our comfortable castle beds around 10pm. Some of us actually had our own turrets, and we all slept as if under a kind of spell.

As the week went on, I noted my mood rising and my stomach sinking. On the penultimate day Rebecca, the chef who had worked wonders for us all week, described the group as "literally glowing". She was right. And it wasn't just on the outside. In our final barre class I glanced over at Andrea, a 45-year-old Italian publicist, and could see that he looked a completely different person from the man I'd met a week earlier. Along with everybody else in the room, he looked leaner, younger and – for want of a better word – happier.

Château de Robernier worked its magic on me, too. My niggling back injury evaporated, I lost significant weight and I felt so positive it was almost as if I'd gained extra muscle around my heart as well as everywhere else. As I stepped out of the castle entrance for the final time, it was with my head held high and my bottom firmly in neutral. I might not have become a black swan, but this fat goose could fly again.

• in:spa retreats run throughout the year in southern Spain and Morocco, as well as Provence. Barre workouts will feature on the following retreats: 14-21 May and 30 July-7 August in France, 17-24 June and 14-21 August in Spain. The one-week all-inclusive breaks cost from £1,895pp. in:spa is offering free flights up to the value of £200 to Guardian readers who book by 28 February. Book on 020-8968 0501, inspa.co.uk, quoting Guardian