Detailed Nighttime Fantasies

I've been married for five years now and we have an open relationship. My wife knows about everything that happens but there's this one thing I haven't told her yet, and it’s eating me up inside. It started off small, just some roleplay stuff online with a stranger who was a little older than me in his mid-thirties. He lived on the other side of the country but we chatted a lot through a private messaging app, sending each other explicit fantasies about different scenarios. Then one night after dinner and some routine Netflix binging before bed, I found myself unable to stop thinking about these fantasies. It wasn't just reading them or imagining what might happen, it was feeling it inside my head as if I were actually there with him. I began to type out a scenario in my mind; we're walking on the beach at night and suddenly he pushes me down into the sand, his body covering mine. I felt the rough texture of the grains grinding against my skin where our bodies meet, cold from the night breeze but warm beneath, and I could almost hear him breathing heavily as he loomed over me. His hands were everywhere, touching me roughly, exploring every inch of me without hesitation. He was in control, but not so much that it felt bad; there was something thrilling about how unapologetic his desire was. I didn't actually type any of this out though, I just let myself live inside the scene, feeling him undressing me slowly, deliberately. His fingers found my breasts, pressing hard enough to leave marks but not bruising, and he pinched at my nipples until they were hard nubs begging for attention. He moved lower, his mouth finding its way between my legs, parting them gently with just the tip of a finger. As I laid there on our living room floor under this fantasy lover’s weight, I couldn't help but imagine what it would feel like to be truly penetrated by him, how he'd enter me, slowly at first, and then harder once he could tell that I was ready for more. His tongue explored every crevice of my body, from the tender spot behind my ear down to where our bodies met in intimate ways. I never sent this scene back out into cyberspace but I've replayed it countless times since. Lying here on the floor under him with nothing but digital bits between us feels as real as anything else in life at this point, and that scares me more than anything else ever has.