As a young girl, Tsering Kyi's favourite days of the year were the eve of her village's annual move to their summer pastures and the eve of their return. The lives of the 30 nomadic households of Tethok, in China's Gansu province, followed the rhythm of the seasons. In the spring they would load their household on to yaks and ride up into the high valleys and hills where their herds would find grass and the children would play with frogs in the lakes and streams. As the winter approached, they would return to lower grazing.

A day before they moved all the heavy items would be packed and sent ahead. The women and children would remain behind, sleeping under the stars, to follow the next day. This was Kyi's favourite time.

"I remember how she was always excited. She loved to sleep outside with her sister and brothers and all the cousins," said a close relative interviewed by the Guardian last week. "Even when she went to school and was a teenager she still came with the family to the pastures in the summer. She didn't like the town so much."

Three weeks ago, in the late afternoon, Kyi, now a 20-year-old student, set herself alight in a vegetable market in the centre of Machu town. Her last acts were to enter a public toilet, take off her traditional Tibetan overdress and douse herself in petrol. She then walked out into the market, ignited the fuel and became the 23rd Tibetan to self-immolate in just under a year.

Every few days in recent weeks there has been a report of another such burning. Since Kyi died seven more have followed suit – including a 27-year-old man who set himself on fire in New Delhi before a visit by China's president, Hu Jintao. The streets of Dharamsala, the Indian hill town where the Tibetan community in exile is based, are full of posters of these "martyrs", as they are known locally. The most recent poster shows a 44-year-old farmer. Few doubt there will be many more.

Map: Machu, Lhasa and Dharamsala

Tibetans in Dharamsala, where the Dalai Lama has lived since fleeing Tibet after a failed revolt in 1959, say the self-immolations are born of desperation in the face of the Chinese authorities' repressive policies. A letter smuggled out from one monastery lists restrictions on Tibetan language teaching in local schools, an increased presence of security personnel and new controls on religious practice as reasons behind the self-immolation of a 34-year-old monk outside its gates two weeks ago. Another factor, it says, is the continuing enforced settlement of the region's nomads.

Chinese officials blame the suicides on "separatist plots" or "criminals". The official Chinese news agency said Kyi was suffering depression following a head injury sustained in an accident. Her relatives, who insisted on anonymity when interviewed, tell a different story.

Kyi was born in 1992, the second child of two nomadic farmers. Her community was the village's 30 households, and her days were spent with the yaks, horses and sheep that were their livelihood. She rarely visited Machu, the town two hours' motorbike ride away where the nearest school and basic health facilities were located. It was a simple, if harsh life. Members of Kyi's family watched women die in childbirth for want of medical attention. Almost no one could read or write.

Change was coming, however. When Kyi was still a young girl, relatives remembered, a new government policy led to each household being allocated a plot of land for grazing and barbed wire divided the high pastures. The old days of allowing the herds to roam freely over the high plateau grassland were over.

Another change was education. Nomad children had never gone to schools in distant towns. But new facilities were being opened, in part to cater for the huge numbers of nomads being resettled in places like Machu. Kyi set about persuading her parents to send her and her younger brother to the Tibetan middle school in the town. An aunt finally convinced them to agree and, aged 11, Kyi started lessons, staying in the hostel attached to the school during term time.

"She did really well. She was starting late like most nomad children but made up for lost time. Her teachers said she was an example to the other kids," said the relative.

Tsering Kyi who died after setting herself on fire this month

But if Kyi thrived at the school, she did not lose sight of her roots. With their herds dwindling due to a lack of adequate pasture, some relatives were now living in Machu town. But Kyi preferred to travel back to be with the rest of her family during the long summer and winter breaks. The contrast with the fast-growing country town where she studied, with its new grids of roads, shops and large numbers of Han Chinese immigrants, was stark.

"She would come back from school and it was like nothing was different, for her or us. She was older and so able to get involved with all the tasks like looking after the yaks, shearing the sheep and so on. And she still had fun," remembered the relative. "She had a wonderful voice and was always in demand in the village to sing at all the festivals."

Such festivals – and their accompanying religious rituals – marked the slow progression of the days. The rites of Tibetan Buddhism were carefully, unquestioningly observed. Even in the summer pastures, a small tent was set up to shelter a sculpture of the Buddha and a picture of the Dalai Lama.

"Religion is everywhere for us. It is just part of our lives," said the relative.

But if Kyi did not show any signs of becoming "political" as a young teenager, she was now an avid reader, hungry for knowledge.

"When she came back to be with her family, she always brought books. When it was dark, when we had all gone to sleep, she would be still there, reading by the light of a lamp," the relative said.

Unpicking the course of events that led Kyi to her self-immolation is difficult. The act itself goes against many of the fundamental tenets of Buddhism and although the Dalai Lama has refused to condone it, he says he understands the reasons behind it. But if Kyi had not actively searched for involvement in activism, she was soon to find herself plunged into its centre.

The spring of 2008 saw the most significant unrest in Tibet for decades. Peaceful protests turned into serious riots in many cities and towns as security forces moved to disperse demonstrators. In Machu police cars and government buildings were burned. In a major crackdown, hundreds were detained, according to Tibetan campaigners, human rights groups and western governments.

But the unrest did not stop, at least not in Machu. Kyi's school became a centre of protest in 2010 when students staged a demonstration calling for more freedom and Tibetan independence. Though dozens were arrested, another protest took place a month later. Then the popular headmaster was fired and at least two teachers detained, provoking further anger. Kyi had found herself in a hotbed of activism.

Following the 2008 unrest there is now an unprecedented level of "political consciousness and Tibetan nationalism", a monk in Dharamsala said last week. He would only speak anonymously as he feared reprisals when he returns to Tibet. "With the internet and mobile phones, everyone now hears about what the Dalai Lama is doing, the protests, every burning. That is a huge change from before," he told the Guardian.

In early January Kyi spoke of the spate of self-immolations and told a close relative that she understood why they were happening. "No one could go on living like this," she said.

Kyi died the day after returning from a month's winter break spent with her family in the winter pastures. She had spent the night at a cousin's home and a friend had given her a lift on his motorbike to school. Kyi did not enter – had she signed the registration book she would have been the responsibility of her teachers and thus have exposed them to reprisals – but headed into the town. One petrol station refused to serve her. A second did not. The last images of Tsering Kyi show her buying a five litre can of fuel.

Hours later Chinese security officials were removing her charred remains from Machu's vegetable market.

• This article was amended on 27 March 2012 to remove details of Tsering Kyi's death that were inconsistent with the Guardian's guidelines on the reporting of suicides.