From the end of 2009 through to 2011, Jackson, a freestyle specialist, was so debilitated by her asthma that one of her ribs would constantly pop out while she was swimming and it got to the stage where London 2012 seemed a million miles away.
It was a far cry from the first part of 2009 when Jackson, who won 400m freestyle bronze at Beijing 2008, beat double Olympic champion Rebecca Adlington twice on the way to breaking a new world record and scooped double silver at the World Championships in Rome.
With her illness and injury problems now under control, Jackson dug deep to book her place at London 2012 in her favoured event - the 400m freestyle - by finishing second to Adlington at the British Gas Championships at the Olympic Aquatics Centre last month.
And, while Jackson admits she's in the dark as to how she will perform when she returns to east London, it's a position in which she's found herself before.
"Over the past few years it has been quite tough with the asthma and I've had a few injuries as well and it has been such a frustrating few years," said 25-year-old Jackson.
"Last year I didn't know if I would make the team in the 400m but I've tried to put everything behind me and it paid off and fortunately I made the Olympics in my favourite event and I'm so excited about it now.
"I'm looking forward to getting back in the pool at the Olympics and racing with Becky - four years ago it was absolutely fantastic with us both winning medals so I can't wait to get back to the Games.
"It feels quite similar going into London as it did going into Beijing. I didn't really know what to expect back then and I didn't expect to get a medal because I didn't have any major medals before so I was just going there wanting to make the final and get a personal best.
"And I think I've got the same perspective again, I don't know how I'm going to swim again and things have definitely picked up this year and my focus is the 400m."
At the British Championships, Adlington, who now looks back to her very best, cruised to victory and by her own admission immediately looked behind her in the hope of seeing close friend Jackson coming home second and inside the qualification time.
Adlington's wish was granted, even if Jackson had to fight tooth and nail to keep 18-year-old youngster Eleanor Faulkner at bay.
But now that she's made sure of her London 2012 spot, Jackson, who will also compete in the 4x200m relay and could yet claim an individual 200m place, believes the pressure is off.
"At the British Championships I was so excited to qualify but I think a lot of people thought it was more a case of relief," she added. "I remember touching the wall and thinking, ‘thank God I've made it'.
"I missed out on it at the worlds last year so to get it back is so good and I'm really excited about racing in the 400m at the Olympics again.
"Last year I was thinking, ‘oh my God, it might not happen!' but for it to be just around the corner is so exciting and I can't wait to get there and race in the 400m."