It sounds to me that when your wife found this other guy you didn't like being some fraction in her life ...you struggled for 7-8 years either in silence or in plain verbal discussions with your wife. There was a snap point,... fuck it,...she's not going to change back I might as well join in. You felt replaced and from that mind set you went about to replace the person who replaced you. And you have to some degree.
The damage done 10yrs ago seems unrepairable ... do you want to be married? Why? Why does she want to remain married ...out of habit ..nice house ...what?
What's her response in taking a secondary role ? How does she define her relationships as far as primary and secondary? ...meaning is the bf a secondary ...or co -primary or is everyone equal?
I'd say you're deep down mono but have adapted to poly out of your circumstances... You maybe happier and healthier finding someone to enjoy that with.
Whats the bf think about all this?