I feel unsettled knowing that she’s seeing other men besides me. Is there any mindset that can serve as a quick fix?

It’s Saito, the columnist♪ I’ve had the same experience, as have people I know. Instead of dwelling on those unsettling feelings, use that time to arrange meetings with new women. Then you won’t end up in that situation. If it still hurts, fill your schedule with even more women. Eventually, you’ll stop feeling anything at all. (It’s a rather interesting phenomenon.) That’s how I got past that point. Sorry to make this about my own experience, but men are creatures who repeat the same worries, so I don’t think there’s any solution other than getting used to it.