She'd been suffering from a number of blackouts – called petits mals – for about six months when she was taken to Birmingham Children's Hospital and told what was wrong with her.
"I was really confused," she admits. "I'd never heard about it before and I'd never heard about anyone else having it, either.
"I didn't want to tell anyone or talk to anyone about it because I was embarrassed in case they thought I was weird."
What made it more difficult was that she had just started secondary school and had to have a total of about four months off in that first year because of the illness.
At her worst, Rosie, who is now 17 and lives in Birmingham, suffered about 60 petits mals a day, which made her go into a strange "trance-like" state for about one or two minutes.
"I called them my dreams," she says.
She tried two different kinds of medication before a third drug, called sodium volproate, gradually helped the seizures to stop. She has not suffered an episode for the past three years.
Rosie said it was thanks to Birmingham Children's Hospital Epilepsy Nurse Specialist Bernie Concannon that she was able to come to terms with her condition and begin to live as normal a life as possible.
"It was really good having Bernie to talk to," she says. "At first I felt lonely and isolated, but she really got to know me and understand me.
"I wanted someone to blame and I blamed the hospital; all the anger was directed at Bernie and the doctors, but everyone was really patient with me and helped me."
Bernie put Rosie in touch with other youngsters of the same age who had epilepsy and they helped each other.
"We emailed each other at first, but we've become friends now," says Rosie, who passed her GCSEs with flying colours and is now studying for her A-levels.
She said her friends and family have helped her to live a normal life because they encouraged her to do the things any young teenager would want to do. Rosie's family in particular have been a huge support to her.
"Even if I could go back, I still wouldn't change a thing," she says. "It's made me a stronger person."
Because Rosie, who is hoping to go to university to study fashion design, is 17, she has had her last appointment at Birmingham Children's Hospital and is being transferred to a general hospital.
"It was weird seeing Bernie for the last time, but I'm really thankful to the Children's Hospital for everything it has done," adds Rosie.