@Soulless67,
This is horrible to me to even read. Well, many of us have had lovers or actual husbands or wives cheat. Sometimes it's not cheating, as many people can and do have open relationships. Some have another culture than what I take as the relatively more strict one in the US and I can understand that, folks thinking there is nothing wrong with having a mistress. In the US, sometimes I think people go way too fast promising all one's love when they have little experience to get what that means.
I suppose it's better you at least know now (geez, your best friend). I'd say you need a separation from these folks - not a vacation, but full cease and desisting.
Also a talk with your family, which I can guess will be hard.
You may need counseling to work your way to getting over this. Or, if you have money, maybe take a wonderful trip to see some of the world.