The family of a young woman who died after an overdose while on leave from a mental health hospital have described in harrowing detail her obsessive-compulsive disorder, saying it took her three or four hours to complete a set of rituals before she could get out of bed each day.

Sasha Forster’s family said she believed she had “evil” inside her and that if she did not complete her complicated rituals, harm would come to her loved ones.

An inquest in Winchester heard that after an earlier overdose her short-term memory was so badly affected that she could not remember the rituals.

A family statement said: “It was awful to see her huddled up on her bed, distressed and crying, convinced some harm would come to us, her family, because she had forgotten a ritual to keep us safe.”

Forster died in March 2017 aged 20. Her family, from Fleet in Hampshire, hope the inquest, which could last up to four weeks, will give them answers about her care, treatment and supervision arrangements.

In a statement read out at the inquest by Forster’s mother, Angela, the family said OCD began to affect her profoundly from about the age of 13.

“Sasha’s rituals took up to 70-80% of her time and thoughts. It took her three or four hours to complete her rituals in the morning just to be able to get out of bed. She then had to continue performing rituals for a further hour or two before she could leave the house,” the statement said.

Her rituals included repetitive touching and tapping and repeating of words and number sequences. As she grew more unwell, she could not stand close to family members.

“By the time she died we hadn’t been able to touch or hug Sasha for five years,” the family said. “Sasha couldn’t allow us to use any objects she had used such as cutlery or pots and pans. Sasha had her own section of the kitchen where she would prepare food. None of the family were allowed to enter Sasha’s bedroom under any circumstances.”

The family used to enjoy sharing popcorn during cinema trips. “As Sasha’s OCD got progressively worse, she stopped sharing our popcorn, she moved a seat away, then a row away, then to a different section of the cinema, until she was at the furthest point from us, hugging her Diet Coke.”

If she did not complete her rituals or if a member of the family broke one of her rules she would weep hysterically for hours, self-harm or run away. “Sasha would harm herself in an effort to prevent harm coming to her family; it was heartbreaking,” the family said.

They said she wanted to recover and was willing to engage with mental health professionals. “Sasha wanted to live a full life, free from the pain and the constraints that her mental ill health imposed on her.”

Forster took a serious overdose on 10 January 2017 and was detained under the Mental Health Act. She was transferred to Farnham Road hospital in Guildford, Surrey, which is run by Surrey and Borders Partnership NHS foundation trust.

The family said her short-term memory was “shot to pieces”. “Sasha remembered her OCD obsessions and her fears about what would happen if she didn’t complete her rituals, but couldn’t remember the rituals themselves.”

On 31 March 2017 Sasha took an overdose while on leave from hospital. She was found by police and taken to hospital, where she died.

Her family, who are being supported by the charity Inquest, hope the hearing will explore the arrangements to manage Sasha’s mental health needs and risk of suicide after her January overdose, as well as the response to events on the day of her death.

The inquest continues.

• In the UK the Samaritans can be contacted on 116 123. In the US, the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia, the crisis support service Lifeline is 13 11 14. Other international suicide helplines can be found at www.befrienders.org.