In an extract published in The Mail on Sunday, the star revealed that she spent most of her childhood in and out of hospital and later suffered a stroke at the age of 18.
"The very first time that something happened was when we were in Epping Forest," she said, recounting how she found out she had a heart condition.
"We were going back to the car after a day out and my dad said, 'Race ya', so we started running but I collapsed. Because I could be quite dramatic and silly, my dad thought I was just messing about. But I couldn't move and I couldn't breathe.
"He realised it was serious so he picked me up and we drove to the hospital. I was afraid and confused. When you're fine and then all of a sudden something like that happens and you don't know why, it can be terrifying."
The then 7-year-old Jessie - whose full name is Jessica Cornish - was told she had an irregular heartbeat and was subjected to a number of medical tests. She was put on beta blockers to try to regulate her heart's rhythm, but they resulted in unwelcome side effects.
"I remember collapsing a lot and having seizures," she explained. "I was a sickly, skinny girl who had a slight green tinge to her skin because of the drugs and who was always in and out of hospital.
"There would be times when I'd be acting normal and then I'd just collapse. It was only then that people would realise I wasn't well. I suppose I was good at covering it up and I know I'm very good at that now."
Jessie J revealed that after overcoming childhood struggles with illness, she watched excitedly as fellow BRIT school students' careers began to take off. Unfortunately, she soon experienced another health setback.
"I was working hard - travelling across London to do my A-levels, modelling, singing in a girl group. I even had a weekend job in Hamleys. And it was at Hamleys that I started to suffer with pins and needles in my right hand and foot."
The 24-year-old admitted that she ignored the sensation "for a few minutes" before she realised it was getting worse.
She said: "I phoned my dad, and he told me to go straight to the hospital. I thought I'd be fine, but then I started getting pains in my right leg. I thought I was having a heart attack - a really slow one. I couldn't breathe in.
"I don't know how I did it, but I managed to get the train home and went to my local walk-in GP clinic. I'd started having really bad shooting pains in my chest, I couldn't feel my right hand, my mouth wasn't moving much on my right side and my right eye was going blurry.
"I sat with the doctor and he said: 'I don't want to scare you, but I'm calling an ambulance, because you've had a minor stroke'. I was like, 'F***, that's dramatic! I thought I had a cold!'
"At the hospital, doctors came in and prodded my leg, but I couldn't feel a thing. It was really, really scary. I wasn't a little girl anymore, and at 18 I was a lot more aware of what was happening - very different from when I was young."
After a two-and-a-half-week hospital stay - which meant leaving The BRIT School four months early - the 'Pricetag' singer worked with doctors to regain her strength and rebuild lost muscle.
"Apart from my right side being achy when I am tired and a trapped nerve I have had under my arm ever since, I'm fine," she said.
"My health isn't as bad now as it was before. Nowhere near. But there are still moments when I have to make sure I'm looking after myself and not pushing myself too hard. I have to be realistic."
Jessie J is rumoured to have been cast in the forthcoming biopic of Britain's Got Talent winner Paul Potts. Her autobiography Nice To Meet You is published by Simon & Schuster and is released on Thursday (September 27).
Watch the video for Jessie J's breakthrough single 'Do It Like A Dude' below: