I cheated on him with his cousin?

I've been upset for a while because I realized my boyfriend wasn’t thinking about a future with me, and I had started to grow emotionally distant. Then one day, I met up with his cousin, and something happened between us. His cousin has been my friend for a few years and is actually the one who introduced me to my boyfriend. It all happened spontaneously, but we both admitted we’d had feelings for each other for some time. Of course, we felt regret afterward and decided not to talk much anymore. Even though I try to comfort myself by thinking my boyfriend wasn’t serious about me, I still feel uneasy. I even feel bad just looking at him. Telling him might hurt him even more. I don’t know if it’s better for him to know or not. What should I do?

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I cheated on him with his cousin?
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