The 27-year-old was contemplating life outside the sport while working for the media at the Olympic Games in Rio.
But Dmitriy Balandin's gold in the 200m breaststroke prompted a rethink, as Jamieson swam faster for 2012 silver.
The Scot will only continue racing if he feels he can compete for a Commonwealth gold medal in 2018.
"Going into Rio, I thought being there in a different capacity, just trying to relax, I was leaning towards finishing," Jamieson told BBC Scotland's Sportsound programme.
"I don't want to be an athlete that hangs on and falls down the pecking order, I want to be competing for medals.
"But the time that I swam in London four years ago [2:07.43] was quicker than won gold [2:07.46] in Rio. The time that won the bronze medal [2:07.70], I've done that eight or nine times in my career."
Jamieson was the only British male to win a swimming medal at the London Games four years ago, but he finished second to Ross Murdoch at the Glasgow Commonwealth Games and has struggled for form since then.
He was not selected for the GB team for the World Championships last year, which prompted him to move away from his Bath training base to Edinburgh University, but Jamieson could only finish fifth in the Olympic trials.
In the immediate aftermath, he admitted he was considering retirement, and will make a final decision shortly, with the competitive fire still burning in him.
"Missing out on the team this year was heartbreaking," he said. "It should have been bread and butter for me to make it.
"I certainly don't have another four years in me, to go to Tokyo [2020], so if I was to continue, it would to looking to the Gold Coast and the Commonwealth Games in two years.
"After having won two silvers in the past, I'd only be going there to win. So the question is if my body's capable of doing that. The honest answer is, I don't know, but psychologically I don't want to be involved in the sport if I'm not going to be competing for major medals.
"I feel like I was very close to a decision before going out to Rio and then seeing the standard of times that won the medals in the 200m breaststroke, I thought 'I'm still there, I'm still capable of doing this. That's thrown the cat amongst the pigeons'.
"I've kept myself in really good condition in the past couple of months. I've been doing a lot of strength training, a lot of yoga, a lot of land-based training, so I wouldn't have a problem getting back to 90% fitness, which I feel is where I've been for the last 12-18 months, but it's that other 10% that's missing.
"The swimmers are all on a break just now, but within the next few weeks I need to make a call. I'm just not able to get the same out of my body as I was a few years ago.
"That's quite a bizarre concept, and still something that I'm struggling to get my head around, but my body is not responding to training the same way as it used to.
"Either way, it's going to be a brave decision. I am leaning towards finishing, because I just don't know if I'm able to get to that same level again."
Jamieson said his highlight from the aquatics events at the Olympics was the emergence of 19-year-old University of Stirling swimmer Duncan Scott, who won two silver medals at the Games.
"There was a great shot of Duncan shaking hands with the great Michael Phelps," Jamieson said.
"Duncan is the slightest figure you'll ever see in a swimming pool, he's got a physique like a canoe, so to see him have that moment with Michael Phelps, after having swum faster than he did in the [4x200m freestyle] relay, was really quite special."
Jamieson was also "quite shocked" at the presence at the Games of athletes who had previously been banned for doping events, in particular the Russian swimmer Yulia Efimova, who won two silver medals.
"Efimova has become a scapegoat and a figurehead for the battle against doping," Jamieson said.
"She's served two doping bans, one which we know was cut short to allow her to compete in the games in Rio, and every time she came out and was introduced to the crowd to compete, she was roundly booed.
"It's an Olympic Games, that's just completely surreal and foreign to me to hear that sort of atmosphere."
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Olympic 2012 silver medallist Michael Jamieson admits he is "leaning towards" retirement from swimming, but will decide his future in the coming weeks.