04/21/2014 AT 05:10 PM EDT
The ugly truth is coming out on True Tori
In a scene from the upcoming docudrama, Dean McDermott
confirms he wasn't happy with his and wife Tori Spelling
's sex life – a problem that led him to infidelity and, in January, rehab
"We would have sex once every two weeks," McDermott, 47, says during an April 9 therapy session. "It wasn't fantastic."
As Spelling, 40, protests, their counselor calls him out.
"Dean, your expectations of what a marriage is supposed to be sexually, it's like a fairy tale," she says.
in 2006 after meeting on a movie set and leaving their respective spouses, Spelling and McDermott have four children together: Liam, 7, Stella, 5, Hattie, 2, and Finn, 20 months.
The actors have never shied away from cameras, sharing their love story on reality shows Tori & Dean: Inn Love
and Tori & Dean: Home Sweet Hollywood
. But Lifetime's True Tori
, premiering Tuesday, tells a very different tale of heartbreak and betrayal.
"I'm insatiable," McDermott admits in the sneak peek. "Sex was an escape, just like drugs and alcohol."
"But I don't understand, like why her?" Spelling tearfully asks of his mistress.
"I wasn't attracted to her," he says. "It was just like a warm body."
The final blow comes after a confession from McDermott already heard in a previous clip
: "I cheated on my wife. That's my worst nightmare."
But Spelling isn't convinced, adding, "Or that you got caught."
McDermott's response? "I don't know."
airs Tuesday (10 p.m. ET) on Lifetime.