Gary Shirley Says Fans Message Him Opinions About Amber's Relationship with Leah: 'People Tend to Forget It's a Show

Gary Shirley is proud of the family he's built in the last 16 years.

Shirley was introduced to the world on then-girlfriend Amber Portwood's episode of 16 & Pregnant. While it was the third episode to air in the series on June 25, 2009, the two actually filmed the series pilot presented to MTV.

"We never signed up for anything," the Teen Mom: The Next Chapter star, 38, tells PEOPLE. "What happened was Amber's brother had signed up for another show about a year before. They reached out to see if they were pregnant, but they weren't at the time. But they mentioned his sister, Amber."

When production followed up with Amber and Gary, "We thought it was a scam," he laughs. "But they wanted us to do a casting video. They sent us a Flip camera and we did the video and sent it back. Next thing you know, they came out and filmed with us."

Amber and Gary's tumultuous relationship came to an end in 2010. They continued to try working together to raise daughter Leah, now 16, but that proved difficult over the years

Over the years, Amber has struggled with legal issues, substance abuse and her mental health. As a result, she hasn't maintained a constant presence in Leah's life. Gary and wife Kristina, who he married in November 2015, have strived to provide a stable environment for Leah.

The last season of Teen Mom has featured Leah speaking on her own experience, particularly in struggling with "abandonment issues" as a result of the rocky relationship.

Gary has grown used to fans sharing opinions and learned that some people can't see past the story that plays out on screen.

"I think a lot of people tend to forget that what they see is a show. There's editing involved and I understand they're going to film and edit things to get the most out of it, but it happens a little differently with that in reality. And they don't understand that," he shares.

"I have people messaging me saying, 'You need to be nicer to Amber,' or 'Leah needs to forgive her mom, it's not Amber's fault.' And you guys see what you see, but we're talking about several years of trying for a relationship that someone else isn't putting the time and effort into," Gary continues.

"I get where people are coming from, and they're not the majority. They might just be in a similar situation, or share something with Amber and that's why they're defending her for certain things. That's how it goes, but overall, our interactions are very positive."

Gary says it's still "a little surreal" that people approach them in public, but it's been good for him and for Leah to see how their story makes an impact.

"Seeing my dad and Amber go through their journey can be embarrassing, because they had a lot of ups and downs — mostly downs from what I've seen," Leah tells PEOPLE. "It's weird to see what they were like at my age, when they had a completely different life and were just starting off on TV."

She continues, "For me, having my life captured on TV has been a blessing. I like being able to share my story and help other people who have gone through some of the things I’ve been through."

Seeing Leah come into her own can be "a little scary" for the dad, but he feels pride in the woman his daughter is becoming.

"Being a father has truly been one of my greatest accomplishments. I'm not always perfect, but I strive to be the best I can be. I often pause to reflect on whether I’m making the right choices for Leah, knowing that I only have one chance to do this right," he shares.

Currently a sophomore, Leah loves "ready, crocheting and baking." She's also excited for what's to come in the next few years.

"I’m looking forward to prom and finishing up school in the next couple years and starting to work on my career in nursing. I’m also excited to get officially adopted in the upcoming year or so.""My hope is that I have succeeded in guiding her. I am forever grateful for her presence in my life, and it is an honor to be her father. Now that she's nearing adulthood, I am struck by how quickly time is passing. I wish it would slow down just a bit so I can cherish these moments," Gary admits.

"She will always be my little girl, and I will always be here for her, committed to my role as her dad."

Check out MTV's celebration of 16 years of the 16 & Pregnant journey on the Thursday, May 1 episode of Teen Mom: The Next Chapter.