Rob Kardashian is convinced Blac Chyna will leave him for a rapper!

Rob Kardashian and Blac Chyna have been fighting for months, partially because he's convinced she's going to leave him for a rapper.

Sources close to the parents-to-be say Rob's never been comfortable with aspects of Chyna's past. For instance, he's brought up her relationships with Tyga and Future. She had to stop listening to certain songs when Rob was around because he once told her point-blank,
"You'd rather date a rapper than a fat Armenian."

TMZ adds that Rob has spiraled back into depression, and has gained back the 50 pounds he lost shortly after hooking up with Chyna ... again hovering around 300 lbs. He's given up on life again, retreating to the confines of his home.

Our Kardashian sources say.. Rob and Blac Chyna have split, and he's been living at his own home for nearly 2 months, almost never walking out his front door.
Rob is also MIA for his own show. Chyna -- who's 8 months pregnant -- showed up for an appearance to promote "Rob & Chyna" and he was a no-show. He didn't even stop by the baby shower that was thrown specifically for him a few days ago.
One big problem ... Chyna is the one who pulled Rob out of his depression, but now the relationship has become the big problem -- they fight whenever they're together and it just deepens the problems.

As one source put it,

"Rob is slipping back into a dark place." We're told the family is praying he "snaps out of it," and they want to do whatever they can to help him.His convos with fiancee Blac these days are tense ... especially if Chyna suggests he work out and eat healthy. He takes it as an insult.