'Slender Man' stabbing survivor's mother offers message of hope to other parents

— -- Since two teenage girls have now been sentenced to spend decades under institutional supervision for the 2014 stabbing of their classmate, the mother of their victim is sharing a message of hope for other families who may find themselves facing the aftermath of a terrible attack on someone they love.

Payton Leutner suffered 19 stab wounds after she was attacked by Morgan Geyser and Anissa Weier with a knife and left for dead in the Waukesha, Wisconsin, woods in May 2014. All three girls were 12 years old at the time. Leutner, now 15 years old, was able to crawl to a nearby road where a passing bicyclist stopped to help her and she survived her life-threatening injuries.

Her mother, Stacie Leutner, remembers caring for her daughter in the hours and days after the attack. She has attended nearly every hearing at each step of the judicial process. For other families who may have children dealing with unspeakable tragedy, Stacie offered some support.

"I understand what you're going through may be the hardest thing you've ever experienced," she wrote in an email to "20/20." "You may be feeling confused, overwhelmed, lonely, or afraid. I felt many of those things over the last three and [a] half years and, at times, it felt like I would never find my way through."

"But if you trust in your strength and believe in your resiliency you will get through this," she went on to write. "It will take time and it will take patience in yourself but you will come out the other side stronger than before. Take time to breathe, take time to laugh. And most importantly be kind to yourself."

When Stacie learned of the attack on her daughter and saw her for the first time, afterwards, she said Payton was "terrified" and "crying" in the hospital's trauma room.

"[Payton] saw me and she put her hand out and I rushed over to her," Stacie told "20/20" in a 2014 interview. "I put my arms around her and I laid next to her and I hugged her. And I said, 'You're gonna be OK. It's gonna be fine.' But I could see that she was covered – her arms and her legs and her abdomen – they were covered in stab wounds."

While Payton's wounds have since healed, in letters to the judge in Weier and Geyser's cases, Stacie and her husband Josef Leutner wrote that the physical and emotional scars left on their daughter are painful reminders of what happened to Payton that day.

Geyser, now 15, was sentenced Thursday to 40 years under a mental health institution's supervision. While she may periodically petition for release from the hospital in the future, she will be under institutional supervision for that time. Though she had pleaded guilty to first-degree attempted intentional homicide, a court then found her not guilty by reason of mental disease or defect.

In December, Weier, now 16, was sentenced to 25 years under a mental health institution's supervision, after a jury found her not guilty by reason of mental disease or defect after she had pleaded guilty to second-degree attempted intentional homicide, party to a crime. She also may periodically petition for release from the hospital after three years.

Now that the sentencing hearings for both Weier and Geyser are over, the Leutners are working to start a new chapter as a family.

"I think Payton, considering what happened to her, is doing marvelously," Kevin Osborne, Waukesha County assistant district attorney and a prosecutor in Weier and Geyser's cases, said. "She's risen above it. She's doing well in school. She has friends. She's social. She's as happy as she can be."

Osborne added, "It's to Payton's credit and her parents' credit that she's doing so well."