EXCLUSIVE

Nikki Bella and John Cena are officially donzo, and that's not based on her reality show ... that's what they're telling friends in real life.

Sources close to the former couple tell TMZ ... the exes are telling friends their on-again-off-again relationship has finally gone kaput because they've grown apart ... figuratively and quite literally.

Remember, while Nikki was filming "Total Bellas," John was tied up in China ... and that distance further fractured their already-teetering relationship.

Our Nikki sources say she also realized while watching Sunday's finale of 'TB' how unhappy she was and hated the feeling of being confused and living in a gray area. Watching that high level of uncertainty reminded her why she ultimately chose not to walk down the aisle with John, despite his pleas.