My Ex-Wife's New Guy and Me

I recently got separated from my wife after six years of marriage. We tried for a while to make it work, but in the end, we both knew it wasn't meant to be. She moved out with her new guy two weeks ago, he's forty, I think she told me once, and they're staying at his place until everything is sorted out legally. It breaks my heart every day seeing him pull up in front of our house with flowers or gifts for her. I've been feeling pretty low since the separation. Work has been tough too; I haven't really had much of an appetite, and I feel like a stranger even at home, which I used to love coming back to after work. The other day, though, something weird happened that I can’t stop thinking about. I was taking out the trash when she called to say she wanted me to come over for a 'talk.' I hadn't heard from her in days and figured it had to do with splitting our stuff up or something similar. She said she missed me, which seemed odd given everything that had been going on. Anyway, we agreed to meet at a park nearby. When I got there, she looked different. Not just dressed differently, but more like... happier? More confident maybe. Her new guy was there too, leaning against a tree with his hands in his pockets, watching us closely. As soon as he saw me, though, the air changed; it felt colder somehow. We started talking about what we were going to do next, and I kept catching her looking at him instead of me, even when she was speaking directly to me. It hurt so much seeing them together like that. But then, after a while, they both stood up suddenly. He gave my wife one last kiss on the cheek before walking off towards his car. She turned to me with this look in her eyes, and I swear it felt like she was checking if I noticed anything wrong or out of place. That's when everything went haywire inside my head; a mix of emotions swirling around uncontrollably. We ended up having sex that day, not because we were trying to get back together necessarily, but just... Because he was there watching us? The idea of him seeing us like that turned me on more than I thought possible. It felt wrong somehow, especially since it didn't feel right for the old reasons anymore. Now I can’t stop thinking about what happened and if I’m losing my mind or if this is a new normal for someone in my situation? How do you reconcile these feelings when there’s another person involved who's technically off-limits but also part of the picture now? Anyone have thoughts on where my head should be at right now?