I know one person who regrets her decision or choices very very much. Daily I'm told. So yes it happens and not just poly vs mono ...but in lots of divorces and break up. Grass being greener on the other side only to find out it's actually worse.
But if you've wanted out for a long time....and keep acting in ways which would be destructive ( drunken tag team, recent affair ) to the relationship why not just rip the band aid off and get it over with. This seems like you want to push him to leave you.