And she worked, until strength replaced heartache and all the anger was gone. It took years.
Now the 29-year-old, whose heated battle with Mary Spencer for a spot on the Olympic team was chronicled in the documentary "Last Woman Standing," is gunning for gold at the Commonwealth Games.
"Those three years (from Olympic qualifying until now) were really hard, I've got to say. For sure they were hard," Fortin said. "But I feel like it's shown me I have mental toughness that I didn't even know I have.
"I was not always strong and on top of things. I'm really proud of what I went through because it really made me better and stronger."
The St. Nicholas, Que., native will step into the ring Tuesday against Phathiswa Thingana of South Africa.
Fortin and Spencer have been two of Canada's finest female boxers for years — Fortin is a two-time world champion while Spencer has won three world titles.
They were also great friends. So it was a bittersweet moment when the International Olympic Committee announced it was adding women's boxing to the schedule for the London Olympics, but only three of the eight weight classes would be contested.
Fortin and Spencer were forced to move up to the same weight class — 75 kilos — and the friends became fierce foes.
Spencer beat Fortin for the spot on the Olympic team, and was touted as one of Canada's gold-medal favourites in London. But once there, she lost her first bout and was eliminated.
Fortin lost her Own The Podium funding. There are shots of a forlorn Fortin working out alone in her Quebec gym in "Last Woman Standing."
Months went by before she could bring herself to watch footage of boxers in her weight class competing in London.
"I don't remember when it was but it was a few months after that I sat in my living room and watched the whole thing," Fortin said. "It was kind of emotional, intense. But I needed to do that, and now it's past me. I really feel like it's past me.
"It's been really hard but I feel like I've gained a lot from it, and now I'm really in a positive mood."
Without a spot on the national team, though, the few fights Fortin has had over the last three years she's arranged herself.
Then Fortin defeated Spencer twice to earn a spot on the Commonwealth Games team. Her confidence grew at a recent international tournament in Poland, where she won three of her four fights.
"I didn't know where I was internationally, what level," Fortin said. "And this tournament, it really gave me confidence, it really showed me that I was at that level, and those three years that we spent at home, we didn't just spend them waiting for nationals. We really trained hard and really believed that we could beat Mary.
"It's funny but I've had some comments about my fights ... people who knew me before, and said, 'Wow you've improved a lot.'"
Canadian coach Daniel Trepanier said he's seen the growth in Fortin, who arrived at the Games with just 15 losses in 130 bouts.
"For sure not making the Olympics in 2012, seeing Mary go to London, made her stronger, it gave her more will to go further," said Trepanier.
He wasn't surprised the seven-time Canadian champion decided to stick at it, resetting her sights on Rio, rather than retiring.
"It's every athlete's dream to go the Olympics," Trepanier said. "And right now, she's in the driver's seat. She's now one of the top athletes in the world."
Fortin and Trepanier hope the IOC will eventually increase the number of women's weight classes at Olympics, but it won't happen before Rio.
"That's what it is for now," Fortin said. "Mary's still boxing. Of course I'm going to face her again at nationals. Nationals is going to be a hard tournament in 2015.
"I know it's not going to be given to me. Now that I've passed the national level, now I've beaten Mary ... I've still got to work. This is just the beginning and I have to be very conscious of that."
Mandy Bujold is Canada's other female boxer in Glasgow. The world champion from Toronto, who boxes in the 51-kilo division, is scheduled to fight Wednesday.
Women's boxing made its Pan American Games debut three years ago in Guadalajara, Mexico. Spencer and Bujold both won gold there.