I have talked about this with my girlfriend, but I don’t want to keep doing that. It makes her feel terribly guilty and sad, and while she wants to help, she doesn’t know how. Neither do I. What should I do?
First, you should know that your reaction is completely understandable in the aftermath of infidelity. In fact, what you’re describing is a common response to trauma. I use the word trauma because while most people can easily imagine (or are personally acquainted with) the pain of being cheated on, what some may not realize is that many betrayed partners experience symptoms of PTSD.
Some of these symptoms are irritability, insomnia, hypervigilance, and difficulty concentrating. People can also suffer from “intrusion symptoms,” such as flashbacks (of, say, walking in on a cheating partner), nightmares related to the affair, physical reactivity to traumatic reminders (like increased heart rate when running into the co-worker), or emotional distress in the face of traumatic reminders (like the mood “disruption” you’re experiencing when seeing him).
The “real issue” here is that the affair was very painful, and seeing your co-worker is a traumatic trigger for the actual issue: betrayal. Part of what makes infidelity so devastating is that it involves multiple levels of betrayal. Yes, your girlfriend betrayed your trust, and the two of you are working through that together. But your co-worker also betrayed you, and this part of the trauma can be especially hard to work through, because most people focus so much on the primary betrayal (between you and your girlfriend) that they don’t take the time to work through—or even acknowledge—the secondary one.
You may be thinking, Wait, I barely know this co-worker. It’s not as if he was my best friend. And to be sure, many would likely say that this isn’t about the other person at all. After all, this person never made a commitment to you. Only your partner did.
But there’s something disingenuous about that line of thinking. The other person involved in infidelity, even if that person is single and available and has little or no connection to the betrayed partner, is complicit in the betrayal. Rationalizations such as “She was unhappy in her relationship—I didn’t do anything wrong” are the equivalent of driving the getaway car in a robbery and claiming not to be an accessory to the crime. “I wasn’t in a relationship with you—she was” is tantamount to saying, “I didn’t commit the theft; I just happily took a share of the stolen money.” These mental gymnastics leave the betrayed partner feeling irrational for having reactions like the one you’re having when seeing your co-worker.
Presumably, your co-worker knew that the woman he was having sex with was your girlfriend. So in addition to the pain of seeing him at work, there’s also the awkwardness of neither of you acknowledging the betrayal. He hasn’t come up to you and said, “I’m sincerely sorry about the pain I caused.” Of course, it’s possible that he hopes you don’t know about it; or that he knows that you do, and he feels too guilty to bring it up.