The 26-year-old has not featured since November after being diagnosed with pericarditis - an inflammation of the fluid-filled sac around the heart.

"I was struggling with understanding how and why it happened," Kirby told the Chelsea website.

"I didn't even have the energy to be frustrated. I had no emotion. It took over my life in a negative way."

Kirby spoke last month of how the condition had hit her "hard" and left her lacking the energy "to even get off the sofa".

She was sleeping for 15 hours a day during the early stages of her illness and has been taking anti-inflammatory medication to combat it.

Pericarditis often occurs after a viral infection and typically causes chest pain and fever.

"I was feeling good with no injuries and enjoying training with the team so it was frustrating at the time," Kirby, who has won 45 international caps, added.

"I know how ill I was and I don't want to go back to that.

"I have more energy and less symptoms but I need to go slowly as if I come back too soon it could be another three weeks out."

Kirby thanks support network

Kirby is not yet back in training with the Blues, but she has returned to the club's training ground at Cobham as she prepares to reintegrate herself into the squad.

"I want to be back on the pitch more than anyone," she said. "I know it is a day-by-day process.

"I also want to thank everyone who has supported me during this time.

"From the doctor at Chelsea to Emma [Hayes, Blues Women manager], the staff, the players and those in my support network.

"It meant so much to me to be surrounded by so many people who cared about me."

Kirby has been restricted to four appearances for the Blues this season after missing the start of the campaign through injury.

She was part of the England squad which reached the World Cup semi-finals last summer, despite seeing her season disrupted by a series of ankle, hamstring and knee injuries.