When Ana Estevez’s ex-husband, Aramazd Andressian Sr., failed to show up with their son for a custody exchange, she says she grew worried and called the police. She says she learned her ex had been found in his car in a park, semi-unconscious and under the influence of prescription drugs -- and their 5-year-old son was missing.

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“In my heart, I always believed that my son was alive, and that Ara had abducted him and was hiding him to get back at me,” Estevez tells Dr. Phil in an exclusive interview airing Tuesday.

During the 72 days that her son was missing, Estevez, who was in a bitter custody battle with Andressian Sr., says she worked tirelessly searching for her child and to spread information in an effort to find him. She says, however, that Andressian Sr. did “absolutely nothing” to help find his son. In fact, she says he was in Las Vegas allegedly attending concerts, skydiving and hiring escorts -- and even changed his appearance.

WATCH: Woman Claims Ex-Husband Who Killed Their Son Made The Child Lie During Custody Battle

Estevez says more than two months later, Andressian was arrested on suspicion of murder.

“My heart just broke into a million pieces,” she says. “Never once did I think that a parent would kill their child.”

Andressian Sr. eventually admitted to murdering the little boy after a day at Disneyland.

On Tuesday’s episode, hear more of Estevez’s story and what she is doing to keep her son’s memory alive. Check here to see where you can watch.

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Woman Describes How Her Ex-Husband Allegedly Murdered Their 5-Year-Old Son