John Cena -- PUBLIC Kiss-Off Before Divorce

WWE superstar John Cena planted a big, fat kiss on his wife on live TV -- just 2 months before he filed for divorce ... but sources tell TMZ ... behind the scenes, he was a wreck over his marriage for months.

Sources connected to the wrestler tell us ... Cena was not shy about discussing his problems at home while hanging out backstage before WWE events.

We're told Cena complained that his relationship with Liz Huberdeau began to deteriorate shortly after they got married in 2009 ... with the arguing escalating in the past few months ... including spats over the remodeling of their Florida home.

One source tells us John has been "miserable" for the past few months ... despite the ringside PDA with his wife at a WWE event on March 5.