I am in a V and my bf is the hinge, I definately can relate to trouble on one side spilling over to the other side. We choose to communicate openly about his other relationship. Being able to talk things over with your mate can do multiple things,
1 It can allow him to vent. Needing no reply or advise.
2 It can allow him to get another point of view.
3 It can allow him to be heard (sometimes this is all people really want/need).
Now this seems one sided but the above, if practiced in a positive manner, can be acheived mostly in 30 minutes to an hour. The benefit to you is the mood will stabalize/improve and you can move on to more interesting topics, ideas or activities.
I understand that some people can't/won't discuss OSO's and their issues. I dont like exclusion or omission. If this is what you prefer then I can't offer any suggestions on how to deal with it. I know its difficult to see my SO sad or upset so I would rather help to ease the frustration than to let them stew in the emotion and possibly ruin my time with them.
Do you have rules in place to keep this from happening?
I find when you restrict or set rules upon communication (especially about feelings)some aren't as clear as what is okay to share and whats not.
Anyone else have any input on this? I am also looking for alternate ways this is dealt with.