Old Habits Die Hard

I'm in my forties now and have been married for nearly two decades. We recently went our separate ways, and I thought I'd be fine. But last week, something unexpected happened. I was staying at a friend's place while we sort things out, and late one night, there was a knock on the door. It was someone from my past, a guy who had always been a bit of an enabler back in our party days before marriage. He said he just wanted to talk, but somehow it turned into something more. I didn't want it to happen, yet part of me felt relieved to be with someone familiar again. Afterward, the guilt hit hard. It was only once, and it's over now, but it feels like an old flame that won't let go. I'm trying to move forward, but how do you close a door when there's always been a crack? I keep wondering if this means I've learned nothing from my years of marriage, or if perhaps I was just waiting for the right moment to look back at all those late nights.