Kendall Santacruze, 20, says the direction of his life was changed by Fitzpatrick's message.
"I'm not even going to lie to you: I was on the verge of getting ready to seriously hurt somebody," he said. "Me and my friends actually had weapons. But Miss Lisa, she stopped us.
"Miss Lisa taught me how to deal with my anger; she taught me how to be in control of myself, (not) let others influence you."
Today, Santacruze helps others at APEX, where he sees young people facing the same struggles that he once did.
"I think to myself, 'That's exactly where I used to be,' " he said. "So I mentor them, I act as a role model to them. The little kids who come up to me, they hug me and they say they love me. It brings tears to my eyes at the end of the day, and it makes me happy to know that I'm actually influencing their lives.
"The way Miss Lisa influenced me is the same way I feel like I'm influencing them."
This peer-to-peer mentoring, and the fact that the majority of the young people that come to APEX are referred by their friends, confirms to Fitzpatrick that she's on the right track. While she's still in the process of assembling measurable data, she said she sees evidence of her impact every day.
"I've seen increases in GPAs, kids are getting jobs, recidivism has gone down," she said, noting that the one statistic she is most proud of is that almost all of the young people she's helped are still alive four years later.
"The successes of APEX are not necessarily going to Harvard or getting out of the neighborhood. (It's) when a kid like Kendall can live (here) and make a conscious choice every day to not be involved in the violence, spread the message of nonviolence, and work hard to get other kids to put down the gun. That's the success."
Fitzpatrick and her family have had to make sacrifices to keep APEX open, trading a five-bedroom house for a two-bedroom apartment. At one point, things got so tough that a woman bought Fitzpatrick groceries with her own food stamps.
But Fitzpatrick said she wouldn't have it any other way.
"At the end of the day, my house and my cars, that was sticks and bricks, steels and wheels," she said. "The kids, they give up everything to be here. That's my inspiration. I can't do anything else but be here."
Want to get involved? Check out the APEX Youth Center website at www.apexyouthcenter.org and see how to help.