Sly Stallone is taking responsibility for his past mistakes ... calling himself a "coward" for once mailing his longtime wife Jennifer Flavin a breakup letter via FedEx.
The legendary actor sat down for an interview Tuesday with Fox News host Sean Hannity — and expressed regret about his decision to send the note to Flavin back in 1994 when the couple had been dating for 6 years.
Sly told Hannity, “That’s the most insincere breakup you can get,” before labeling himself a “coward” and stating, “You put it down in words ‘cause you don’t have the guts to do it face to face.