My Secret Fantasy and Guilt

I've always been drawn to this kind of thing since I came out as trans. My partner is everything I want, loving, supportive, kind, but when it comes to sex, there's a particular fantasy that's haunted me for years. It started small at first, just imagining something different during vanilla sex. But over time, the more I explored my desires alone in secret, the stronger this urge grew. Now, here's where it gets complicated: I fantasize about being tied up and whipped by a stranger, someone who doesn't care for me but sees me as an object to use. It feels dirty, wrong, like betraying not just my partner, but the person I've become since transitioning. We had our two-year anniversary last month, and things were great. Yet during sex one night after drinks with friends, a friend's accidental joke about rougher stuff triggered this old fantasy in me more strongly than ever before. My partner was into it too, wanting to spice things up, but that's not what I wanted then, no, what I wanted was someone else. I tried pushing it away, focusing on the pleasure between us. But later, after we finished and were lying next to each other, my mind kept drifting back to this secret fantasy of being used and controlled. I felt dirty for even thinking about it in that moment with her, but I couldn't shake it. So here's the thing, how do I come clean? What if she doesn't understand? We have an open relationship, so there are no hard lines on what either of us can explore alone or even together. But this...this is different for me. This feels like a betrayal to who I am now. Tonight we're going out again and I know it will happen eventually. The fantasy won't go away just because she's there next to me. What if she finds my secret stash of toys, videos? What then? In the end, when you get down to it, my body, her partner, but tonight I want a stranger's hand on me, pulling at my hair and pushing me into something new. It's not about her or him, just this need that won't let go of me.