Dylan Hartley says his World Cup omission left him feeling like he had died and it continues to keep him awake at night.
But the Saints hooker is now determined to focus on life at Saints as he targets a comeback from the knee injury that has sidelined him since last December.
Hartley had dreamed of ending his England career by guiding his country to glory in Japan.
But he was left out of head coach Eddie Jones' 31-man squad for the tournament having failed to overcome what Saints boss Chris Boyd has labelled 'a grumbly' knee problem.
Speaking at a Dove Men+Care event, for the first time since his World Cup hopes were shattered , Hartley said: "Health-wise, I'm good. Healthy. Mentally stable. I'm in a good space, actually.
"When the World Cup squad was announced, it was almost like I'd died - friends getting in touch and seeing if I was OK.
"Ultimately, the way I looked at my whole rugby career and how I wanted it to pan out, and how it was going, was that this would have been the ultimate send-off, the dream finish to an international career.
"The reality is you pick up an injury.
"For four years we have talked about winning the World Cup with Eddie, and I have led that group, been a part of that journey, so to fall at the final hurdle... I don't know the feeling.
"I wake up most nights thinking about it. It doesn't sit well with me, but I have come to terms with it. You have to crack on."
Hartley, who has been capped 97 times, still harbours hope of resurrecting his England career.
The 33-year-old has not featured at any level of rugby since representing Saints in a Gallagher Premiership game at Worcester Warriors almost nine months ago.
While he was unwilling to provide specific details about the issue with his left knee, referring to his medical records as 'private', the New Zealand-born player admits he aggravated the problem by attempting to rush back.
He has since spent a fortnight in Philadelphia working with strength and conditioning specialist Bill Knowles, whose previous clients include Andy Murray, and feels that treatment was beneficial.
"I tried an (initial) approach with my knee, a very aggressive approach, because I was given some deadlines to meet, and the aggressive approach did not work," said Hartley.
"I basically ended up in a worse position for trying to come back too early. Ever since that happened, I knew I wasn't going to make the (World Cup) deadline.
"It was around the start of the camps, when they first got together around the start of July. I tried getting back for that and basically put myself in a bit of a hole physically and mentally.
"Everyone asks when you are coming back but I have learned that you do not put a time frame on it, because when you are 10 days out from it and you know you're nowhere near, you can play these games in your head and it's tough.
"At the moment I'm building my tolerance to be able to run every day and participate every day."
In his enforced absence, Hartley has backed England to win the World Cup, which begins on September 20.
He also joked that should that happen he will lead the celebrations sporting the kit he wore on his last international outing, against Australia in November 2018.
"I genuinely think England (will win), they've got as good a chance to go and do it," he said.
"The worst thing for me is to think if my leg wasn't f***** at the moment I could be there with them, winning a World Cup. It's hard to swallow, really.
"I hope they go and win. I'll get on and be a glory supporter - remember me guys?
"I'll be hanging off the side of the open-top bus, in my 97th (appearance) shirt."
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