welcome here.There are some great threads and answers about emotions and how to learn to deal with them here.
when you say they don't get to see each other very often - how often is that?
A few times a year? She sees him when she goes to her parents house (Baltimore) and sometimes he comes here.
Some of it does, yes. With him she has no responsibility. No stress. With me it's working two jobs, a special needs son, and responsibility.
Welcome! If I might ask, what do you get out of the arrangement? It doesn't have to be another relationship, if that's not what you want, but does she take care of your child for a weekend so you can pursue a hobby, etc.? If one partner feels like a poly arrangement only has benefits on one side, it can definitely cause some resentment.
How long has your wife's relationship been going on, and how did the two of you decide she could pursue a poly relationship?