This is hard but you gotta do the right thing for yourself,  break up with her. Not only did she cheat on you, she did it multiple times. It doesn't matter if someone close to you died, it's not an excuse to forget you have a fiance and fuck some random dude she met at a funeral multiple times. You gotta walk away man, that feeling you felt when you saw her in the morning is what you will feel everyday for the rest of your life if you stay in this relationship. Call off the wedding, let your family and friends know exactly what happened before you break up, then break up with her. 6 years is long but you still have your life ahead of you.