When I first started dating my girlfriend I asked her about past boyfriends and she said she hadn’t met the right guy yet. After dating for nine years I found out about a past boyfriend and looked through her emails. I found she dated her married boss for three years. She broke up with me for looking and judging her about it. I feel like she lied and she thinks it was none of my business. We’ve been broken up for five months. She’s reached out but I can’t get over my anger or disgust that she was someone's mistress. Am I a bad person?
Still Angry And Disgusted
"Haven't met the right guy yet" ≠ "Haven't met any guys ever."
Almost everyone has done something and/or someone that they regret doing—although it's possible she doesn't regret dating her married boss and doesn't need to regret fucking her married boss. Sometimes people have affairs for all the right reasons and their affair partners doing good, not bad. It's also possible—it's perhaps likelier—she felt pressured into having an affair with her married boss or it went on longer than she wanted it to because he was her boss and she stuffed it down the memory hole after it was over because she wasn't proud of it and wanted to forget it. It's also possible she didn't tell you about this relationship when you asked because she intuited—correctly, as it turned out—that you are, in your own words, a bad person, e.g. the kind of guy who'd punish his girlfriend for having a sexual history, for having made mistakes, and for deciding to keep some things private. (Not secret, SAAD. Private.) In other words, she correctly intuited that you would punish her for being human.
And she didn't even lie to you! Her boss clearly wasn't "the right guy," seeing as he was married and her boss, and seeing as that relationship ended before you two met nine years ago. And... well, kinda looks like she still hasn't met the right guy.
Finding out about a past boyfriend doesn't give you the right to invade your partner's privacy and dig though their ancient emails. She was right to break up with you.
To be frank, SAAD, I don't want to help you get over your anger or disgust—not that you asked me to help you get past those feelings. It almost sounds like you want your anger and disgust affirmed... so I'm going to go with that and affirm the shit out of those feelings: Stay angry! Sstay disgusted! Not because those feeling are valid—they're not—but because they prevented you from taking her back when she reached out. She may not realize that yet, SAAD, but she's better off without you and deserves better than you. So I want you to stay angry and disgusted, dude, just long enough for her to realize it.