"The life that Kristoff led, a lot of people don't know what was happening at the end of his life," Mia St. John tells PEOPLE

Mia St. John is ready to share her story.

In an exclusive interview with PEOPLE about her upcoming book Fighting For My Life, the five-time world boxing champion opens up about how her ex-husband Kristoff St. John‘s sudden death and the loss of their son to suicide led to her passion for spreading awareness about mental health.

“I’ve been in this fight for reform of our mental health system for so Iong,” says Mia, 52. “Kristoff and I lost our son in a mental health facility that was found guilty of neglect. Then when I uncovered a lot of the facilities in LA County, I discovered that this was going on everywhere because there have been many deaths. My son wasn’t the only one. So I’ve been on that vision.”

The two-time Emmy Award-winning actor died at age 52 on Feb. 3. It was later determined that his death was caused by heart disease, which was exacerbated by alcohol use.

Kristoff and Mia’s son Julian, 24, died by suicide four years earlier on Nov. 23, 2014 following a lifelong battle with mental illness. He was reportedly found unresponsive in the bathroom of a Long Beach, California, facility.

Mia St. John
Mia St. John

“When Kristoff lost his life, he was also neglected by the facility that he was put in,” she continued. “He was let go while still being suicidal. Then, my mission just completely skyrocketed. And I just want the public to be aware of what’s happening.

She continues: “A lot of people don’t know what was happening at the end of his life. Not just with the depression and the guilt over his son, but all the circumstances surrounding his life and the last days of his life. So much has been swept under the rug.”

Just two weeks before the Young and the Restless star’s death, he publicly mourned the loss of his son on Twitter. On Jan. 21, St. John retweeted someone’s post about losing a child that read, “Grieving the loss of a child is a process. It begins on the day your child passes, and ends the day the parent joins them.”

“Never a truer word was spoke,” St. John tweeted. “Thanks for posting this.”

20th Annual Daytime Emmy Awards
Kristoff and Mia St. John
| Credit: Ron Galella, Ltd./WireImage

According to Mia, Kristoff was struggling in the days leading up to his death.

“Everybody knew what was going on in his circle, but I feel like there was such a big attempt to cover everything up, which was not to really his benefit and he was so eager to get out of the hospital and get back to work from fear of what would happen if people found out he had been hospitalized,” she said. “He needed to stay in that hospital. And if he had stayed in that hospital as we had intended, and as our attorney had requested, he’d still be alive today.”

And Mia admits that she has been struggling to come to terms with the losses she’s experienced as well.

In March, Mia revealed that she relapsed after three decades of sobriety.

The 42nd Annual Daytime Emmy Awards
Kristoff St. John with daughter Paris St. John
| Credit: Sean Smith/CBS via Getty

“I lost my 30 years’ sobriety,” she previously revealed to PEOPLE. “I’m ashamed, believe me. I had 30 years of sobriety, I thought I was the s—. Like I was queen of mental health. I’m a mental health advocate. I felt as small as you can get. My ego was crushed. But I feel like I want to look at the bigger picture and my higher self. It’s about the good of all. And if I can help someone out, and say if there’s just nothing it doesn’t mean that I’m a weak person. I’m just an imperfect person and we all have to find a way to pick ourselves up and carry on.”

Following Kristoff’s death, Mia was hospitalized twice.

I’m out of the hospital. I’m sorry if I scared anyone,” she shared on social media. “Let’s plz focus on the wellness of his precious girls Lols & Paris.”

“They love their father so much, the beautiful, loving, funny soul that he was/is,” she continued, adding a green heart emoji. “#MentalHealth is all we have, nothing else is important.”

While Mia says she’s overcome with grief, she’s doing her best to move forward.

“We’re just really struggling,” she tells PEOPLE. “I’ve just been telling my daughter, that we have to go on, he’d want us to be happy and to celebrate together. And that’s what we have to do, even though he was always with us, even though we were divorced, we spent holidays together. It’ll be a burst. It’ll be difficult.”

Fighting For My Life will be released in 2020.