Play video content EXCLUSIVE

John Cena has just faced his biggest fear and lived to tell the tale ... and TMZ has obtained video of nightmarish ordeal.

No, we're not talking about another match against CM Punk -- his girlfriend Nikki Bella just dropped a massive bombshell on the WWE champ ... asking him about the future of their relationship.

It all went down while the two were filming for Bella's new reality show on E! called "Total Divas" which debuts July 28 ... and Cena is clearly uncomfortable when she asks.

Cena tells Nikki that "both marriage and family are difficult obstacles" for him -- but eventually ends up coming clean about his feelings towards marriage ... including his own failed attempt.