I have been where you are and I know how much it hurts. (Only difference is I didn't have a husband; it was just the three of us.)
The way I coped was by putting one foot in front of the other, keeping my goals in mind, and clamping down on my emotions around the parties in question.
I dare say that I will be surprised if their marriage withstands this. In our case poly magnified the issues between my couple. It was easy to blame the poly circumstance, but that was never the issue. A year after I left, they separated and are in the process of divorcing. Let me tell you though, it gives me no satisfaction.
I am sorry. :/