Parents Who Lost Kids in Sandy Hook Share 'Bittersweet' Feelings About Survivors' Graduation (Exclusive)

“Every one of these milestones just causes you to step back and think about where we would be ... if things had gone differently,” Michele Gay tells PEOPLE

The surviving first graders of the Sandy Hook Elementary massacre are now graduating high school. And while the milestone is a celebratory one, it's stirring complicated feelings of both joy and sorrow for the students and the parents of the 20 classmates who were killed 12 years ago.

"Every year there are different milestones and every year there's a change of seasons and holidays, all of those things — but this one's a doozy. This is a tough one," says Mark Barden, who lost his son Daniel, 7.

"Of all the other things that have been hard, all of the other life experiences in life itself that have been stolen from him, high school graduation is a very special moment in one's life," Barden tells PEOPLE, "and it's so hard to get my head around it."

In recent days, PEOPLE spoke with 10 of the graduates who survived the shooting, as well as parents of the children who died.

“It’s part of your DNA. You’ve been changed forever,” says Michele Gay, whose 7-year-old daughter Josephine “Joey” Gay was killed. “You just have to learn to live with it and move around it.”

For some, there is solace in seeing how life has gone on: Scarlett Lewis' son Jesse, 6, is said to have helped save classmates who are entering young adulthood.

High school graduation marks "a great day, because those kids that he saved lived for a reason," Lewis says. "They have an incredible opportunity to use their experience and all that they've learned through living through that tragedy and use it for the benefit of others. I feel like that's why we're here ... to help others."

Below, parents of the children slain at Sandy Hook reflect on their kids' memories, how their own lives have changed, what graduation day means to them and their hopes for the class of 2024.

For more from these parents and other community members, read next week's issue of PEOPLE, on newsstands June 21.

Related: Sandy Hook Survivors, Now 17 and 18, Reflect on Trauma While Growing Up and Their 'Happy-Sad' Graduation (Exclusive)

Scarlett Lewis, mom of Jesse Lewis, 6

Though she sees Jesse’s classmates graduating from high school as a “great day,” Lewis says attending the ceremony — held on Wednesday, June 12 — could have been "too painful.”

Instead, she and her son JT did something "to honor Jesse on our own on that day,” she says.

It was 12 years ago that Lewis was first approached by police, informing her that her 6-year-old had died after urging classmates to run.

“It's been really meaningful for me that the kids have acknowledged Jesse's courage,” Lewis tells PEOPLE about the students who recalled Jesse’s bravery in his final moments.

Lewis — who started the Choose Love Movement in honor of Jesse — says it has been an “11-year-journey to find the solution" to societal problems like violence, substance abuse and mental health problems, but it is something she is willing to fight for for the rest of her life.

“We take what we've learned through our lived experience, both good and bad, and use it to help other people. That's the world I want to live in. That's what we call choosing love,” Lewis says.

“We all have the capacity for the courage that Jesse showed," she says.

Michele Gay, mother of Josephine “Joey” Gay, 7

“Every one of these milestones just causes you to step back and think about where we would be, where we wish we were at this moment in time, if things had gone differently,” Gay tells PEOPLE. “It’s a lot to think about.”

With two older daughters graduating from college this year, Gay says celebrating is hard but that Joey “would always want us to focus on the joy of life.”

“She was just a force in this world and in our lives, and we all just kind of orbited around her,” she says. “Her laughter was ever-present. She was always smiling.”

Gay — who co-founded the organization Safe and Sound Schools with Alissa Parker, another mother who also lost her child during the massacre — continues to push for a better tomorrow while carrying her sadness with her, she says.

“It goes with you wherever you go,” she says. “You just have to learn it's part of you now. And if you do, you find that you're really very strong.”

Mark Barden, father of Daniel Barden, 7

"You can't not think about what our little Daniel would be doing now, what he'd be looking like," Barden says, adding, "Graduating from high school ... It's like a rite of passage that everyone deserves to participate in."

"I'm always thinking about Daniel," says the father-turned-advocate. "What would he be doing today? What would he look like?"

After the loss of his son, Barden went on to co-found Sandy Hook Promise, an organization that he says so far has averted 16 school shootings through their training program for students and has prevented over 600 imminent suicides.

“I never lose sight of the privilege that I have of being able to honor my little Daniel with this work,” Barden says. “Sparing other families this lifetime of pain and grief by having to lose a loved one to preventable gun violence.”

Related: Six Years After Sandy Hook, Dad Can't Stop Remembering His Last Moments with His Little Boy

Nicole Hockley, mother of Dylan Hockley, 6

“I focus so much on my work as a way to not kind of deal with some of the harder things that I'm just reticent to really face,” says Nicole Hockley, who lost her son Dylan and co-founded Sandy Hook Promise with Barden. “Dylan would have turned 18 this past March. He should be graduating from high school. And yet, for me, he's still 6 years old. It's just so hard to realize how much time is passing, and how quickly it's passing.”

Hockley tells PEOPLE that the graduation of Dylan’s class is “bittersweet” but that she is “ so proud” of her son’s classmates.

“I was at the high school [recently] for their awards night, handing out two of the Dylan Hockley Promising Futures Scholarships, and I left shortly after,” she says. “I'm so proud of these kids and can't wait to see what they do next, and thinking about how much they've already been through and how resilient they are, but it was also just very hard because I'm seeing kids that I still think of as 6-year-olds in Dylan's classroom.”

Related: Sandy Hook's Surviving 1st Graders Graduate High School: 'Monumental' but Tinged with 'Sorrow' (Exclusive)

Hockley has a surviving son, Jack, whom she refers to as her “living heart,” and still talks to Dylan every day.

“I still have his urn in my bedroom. I still kiss it every single morning and every single night and tell him I love him and I miss him. If I have to wrangle through a difficult decision or something that I'm nervous about, I kind of ask him to look after me,” she says.

She says Dylan, who had autism, left an everlasting effect on the world. “One of his repetitive moments with his autism was to flap whenever he was excited, and I asked him once, ‘Why do you flap?’ and he looked at me and said, "Because I'm a beautiful butterfly.’ ”

When it comes to curbing violence in the world, be a little more like Dylan and start a "butterfly effect," his mom says.

“It is these small changes, like the flapping of a butterfly's wings, that can accumulate and cause larger changes.”

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