For Eric Erdman, life hasn't always been easy.

He was born 25 weeks early and weighed less than 2 pounds. He had learning challenges growing up, and was a victim of child abuse and bullying.

Then at 16 years old, Erdman was diagnosed with anaplastic ependymoma, cancer in his brain. Now, at 19, he's been told he has five months to live.

"Cancer is the best thing that ever happened to me," Erdman told "Good Morning America." "It gave me my voice. Sharing story with cancer allowed me to have a platform in my community."

Giving a voice to others, particularly children, has become the primary focus of Erdman's life. It's what he has set his sights on in the last days of his life, despite four brain surgeries, three rounds of radiation and a half-cycle of chemo.

Two years ago he formed the foundation Give a Child a Voice. It has three pillars: bullying, child abuse, and life-threatening childhood illnesses. The goal, through community programs and partnering with educators, is to encourage children to speak out when faced with difficult circumstances.

Speaking out is something Erdman, who described himself as a "shy and introverted kid" wasn't able to do himself.

"I felt like I was on an island," he said. "I bottled everything inside [when it came to the abuse and bullying]. But I've come to realize it's more painful to be silent."

"Break the silence" is the mantra of Give a Child a Voice.

The foundation also strives to give kids safe spaces. Erdman used his wish from the Make-A-Wish Foundation to have a brand-new gym installed in not only his own high school, but that of his school's rival high school as well.

"It was where I went when I needed an out," he said. "Working out finally gave me the confidence to tell my brother and my mom what was happening to me."

Now he hopes those gyms, along with the foundation, will continue to empower kids to speak out rather than suffer alone. He even hopes they will reach out to him directly if they need someone to talk to.

"People contact me personally all the time," Erdman told "GMA." "Maybe I haven't been through exactly what they have, but I'm here to help if I can."

The teenager hopes he can use these last months to help more children "become the author of their own story," as he did.

He told "GMA" his current favorite quote goes something like this: "We are all alike, books on a shelf, some with more chapters than others, but every year there are 365 blank pages just waiting to be written."

And while his book may "be kind of tiny," Erdman said, "I may not be able to control my illness or if my tumors grow. But I found my voice a few years ago, and now I am the author of my own story."