I don't really have much of anything to add to the excellent advice that you have already been given in this thread.
I do empathize with you as two years ago I found myself in almost exactly the same situation. I was cheating on my husband with his best friend - although no actual sex was involved. I had convinced myself that MrS wanted a DADT policy and was convincing enough that Dude almost believed me (even if he didn't agree - he was always a proponent of full disclosure).
You can read about my experience in my blog here (the "Journey" link in my sig). (The JACKASS part starts around message 21 with the preamble in message 19). Although I haven't finished the story there I can tell you that it does, so far, have a happy ending. MrS, Dude, and I have been living as a cohabitating Vee for 17 months now - and things are going well (which, at one point, didn't seem like even a remote possibility - I am, however, the LUCKIEST girl on the planet...)
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe