Nick Cannon -- I'm The One Who Left the Relationship

Nick Cannon is the one who decided to bail on his marriage -- not Mariah -- sources connected with the couple tell TMZ.

Nick has made it clear to all involved ... he's deeply concerned about Mariah's emotional state and is ending the marriage for the well being of their kids. He believes the environment around Mariah is toxic and fears for his kids.

We're told Nick believes his kids have already shown signs of emotional upset living with Mariah.

Our sources say Nick's MO is simple ... he wants to create an environment separate from Mariah that gives the kids a safe haven from the "chaos" at her home.

Nick is also telling his people he's deeply concerned about Mariah's mental health and believes her people don't care about anything but her ability to make money and pay their salaries.