The mother of a mentally ill daughter who killed herself last month has revealed how her condition worsened over years and eventually drove her to suicide. Doris Fuller's daughter, Natalie, 28, suffered from bipolar disorder and psychosis, and died when she stepped in front of a train in Baltimore, Maryland, last month. And in the wake of her death, Fuller has written about how her illness, which went undiagnosed until her early 20s, and said she was 'the bravest person I have ever known'. Scroll down for video . Opening up: Doris Fuller's daughter Natalie, left with her mom, killed herself last month - and now Fuller has opened up about how Natalie's bipolar disorder and psychosis led to her death . The mother and daughter pair had shot to fame in 2004 when they collaborated on a parenting guide based on Natalie being totally honest with her mother about whatever she was asked. Bestseller: Natalie opened up to her mom - and the result was this popular parenting book . In a long article about her daughter's life for the Washington Post, Fuller wrote how their famously close relationship was changed for good when Natalie had her first psychotic episode at the age of 22, after she had left for college. Fuller wrote: 'In the span of a few weeks she went from being a dazzling young adult with the world at her feet to a psych-ward patient with an arrest record.' Summarizing her last years, she said: 'My daughter lived more than six years with an incurable disease that filled her head with devils that literally hounded her to death, and she did it while laughing, painting, writing poetry, advocating and bringing joy to the people around her. 'She was the bravest person I have ever known, and her suicide doesn’t change that.' And though medicines and proper treatment gave Natalie her life back for a time, she eventually decided to stop taking her meds, which brought her troubles back, even more acute than before. In a mental health documentary video filmed in 2014, Natalie described the worst lows of her condition, in which she said terrifyingly real voices would tell her to hurt herself. She told the cameras: 'I believed that everything happening to me was real, and I thought that I was acting fine. 'But my mom and my brother and everyone around me could see something that I couldn't see. 'Even though [the voices] were telling me to hurt myself and do horrible things to myself, I still listened to them because I had a relationship with them. I thought they were real.' Although Natalie, a keen artist who sculpted, painted and wrote poetry, often seemed fine while on medication, Fuller told how her future seemed a little bleaker every time she relapsed. Difficulties: Natalie (left in 2014) was not diagnosed with a mental health problem until she was 22 - before then she had a regular childhood (right). She told how as she grew older voices started telling her to harm herself . Growing up: As a teenager, Natalie's no-holds-barred honesty helped make her mom a bestseller . She wrote: 'If she even inadvertently missed a few days of medication - even while receiving therapy and other forms of treatment - the demons would return, and one of the first things they would tell her was to stop taking her medicine. 'The second thing they would tell her was not to talk to her mom, the most powerful other influence in her life. 'Each time she obeyed and relapsed, she plunged into a longer free fall, hitting the ground harder, recovering more slowly and returning at a lower plateau.' According to the article, the final, ultimately fatal, change came last fall when Natalie concluded that the drugs were not helping her at all, and gave them up for good. Fuller was not specific about what happened in Natalie's last weeks, though in a post on a tribute site to her daughter, she mentioned that she had holidayed in Nicaragua with members of her family in February. Fuller, the executive director of the Treatment Advocacy Center charity has said since Natalie's death that she wants her 'legacy' to be increased understanding and compassion for the those who have mental illnesses. Treatment: When she featured in the video, Natalie told how she was living in a treatment facility and caring for animals in a bid to get better . Missing her: Natalie would have been 29 last week -pictured above is a cake her mother had made for her .
Natalie Fuller, 28, stepped in front of a train in Baltimore last month .
Mother Doris shared an account of her life this week in heartfelt article .
Told how she was diagnosed with psychosis and bipolar disorder aged 22 .
Pair wrote bestselling book, Promise You Won’t Freak Out, in 2004 .
Featured nationally on network news and was featured on Oprah .