Britney Spears just dumped her boyfriend of 18 months ... after finding out there's a video of him cheating on her that's being shopped around Hollywood. We're told David Lucado is seen in the video making out with a woman and dancing with her. We're told the video was shot in early August. Sources tell us as soon as Britney got word about the vid ... she broke up with David. She just tweeted, "Ahhhh the single life!"