Caught in My Own Web of Lies

So here I am, a stay-at-home parent for the past three years. My partner, let's call him Alex, works long hours at his job and we've been dating casually since we met four years ago. Nothing exclusive but nothing casual either, you know? We have this understanding that if one of us starts to feel like it’s more serious than the other wants it to be, we talk about it. About a month back, I started feeling things for Alex that I hadn’t in a long time. The spark was gone and I wanted it back, something fresh, something exciting. But here's where I screwed up: instead of telling him how I felt, I thought maybe if I pretended to be into other people, he would get jealous and want to spice things up. I joined this online dating site specifically for married people just so I could have a pretend fling going on the side. I met a guy named Jack and we started messaging each other every night after Alex left for work. We never met in person, but our conversations got more flirtatious over time. There was one day when Alex had to stay late at work and I couldn't help myself, I invited Jack to come over. We didn’t do anything physical, just cuddled on the couch and watched a movie while texting each other. It felt harmless, but it also made me feel guilty for wanting something my partner couldn’t give me then. The problem was that when Alex came home late from work that night, I could tell he noticed something different about how close Jack and I had become. Since that day, I’ve been avoiding any direct questions from him about what we talked about or where we were going with things. But here’s the kicker, after one of our conversations with Jack last week, I realized there's someone else in his life now too, and he said it feels like an open relationship but nothing official. I’m caught up in my own web of lies now, feeling both relief that Alex won’t feel guilty about not being exclusive and anxiety that I might lose him if this comes out. The guilt is eating me alive; I can see how much pain pretending to be into someone else causes when you actually love the person who's there for you. So here’s my confession: It feels like a stupid mistake in hindsight, but it was just an attempt at regaining something that felt lost with Alex. What do you think? Am I really the asshole?