Cena says he was blindsided by the breakup -- and blames it on an "unfortunate set of circumstances" ... though he wouldn't elaborate on the specifics.
Cena says he's been depressed -- and he's been reflecting a lot ... and has decided to change his stance on fundamental things in order to make the relationship work.
"I love her. I want to be with her. I want to make her my wife. I want to be the father of her children. I just want us to work."
Cena has been pretty vocal about the fact he didn't want kids -- but says he's a changed man now.
The good news for Cena ... the WWE star says he's still talking to Nikki ... so, maybe some hope?