It wasn't another woman that got between Mark Sanchez and Eva Longoria ... it was 52 other men ... TMZ has learned.
Sources connected to the former couple tell us ... they broke up because Mark was impossible for Eva to be around because he was in such a crappy mood over the NY Jets' horrible season.
As one source puts it, "It's hard to maintain a relationship when one person is upset all the time."
FYI -- the Jets are 3-4 and coming off another heartbreaking loss to Tom Brady's Patriots.
Despite the split, we're told the two are serious about remaining friends -- and have nothing but respect for each other.