A Secret My Partner Won't Understand

I've been in an open relationship for a little over a year now. I'm studying towards my master's degree and my partner is supportive of this arrangement, even though it was initially difficult for them to grasp the concept of non-monogamy. We have clear boundaries and we respect each other's choices. A few weeks ago, something happened that has been weighing heavily on me ever since. I met someone online who shared similar interests in literature and art. We started chatting frequently, which eventually led to us discussing our relationship statuses more openly. She was intrigued by the idea of an open relationship but emphasized that she's never done anything like this before. One evening, after a particularly draining day at my university lab, I found myself feeling unusually lonely despite knowing it wasn't rational given how well-supported I feel in my current situation. This new person offered to talk me through some difficulties I was facing with one of my projects. We ended up video chatting for hours, exploring each other's minds and interests rather than just discussing work. As the conversation deepened, there was a shift where we began to share personal fantasies, nothing explicit at first but suggestive enough that it became clear both of us were interested in taking our relationship beyond just friendship. I hesitated initially; I genuinely care about my current partner and value the stability and trust between us. However, as weeks passed, these conversations intensified until one night we found ourselves sharing intimate details over a phone call, whispering softly about what it would be like to meet in person soon. It was consensual, of course, she wanted to understand me better on all levels and I couldn't resist the allure of something new despite knowing it might hurt my partner. This isn't an issue with non-monogamy or even an ethical dilemma regarding fidelity; instead, it's about a breach in honesty. My partner trusts me implicitly, but this secret feels like a violation of that trust because I know she would never want me to engage outside our agreed-upon parameters without discussing and agreeing first. I'm ashamed and hesitant, not because I regret what happened or doubt my choices, but because I've betrayed the openness that we both cherish in our relationship. And yet, every time I think about telling her, my heart aches at the thought of losing this emotional connection before it even fully begins.