Engaged in More Than Engagement Rings

So I've been engaged for a little over a year now. We met online, dated for about six months before things got serious. My fiancée is great, really sweet and kind, and we have this whole narrative that’s been built up around our relationship being something special, endgame material, you know? But here's the twist: I started dating my ex again after three years of being apart. We never told anyone about it. We'd meet up every few weeks when she came into town for work-related stuff, but we were always very discreet and careful. It was all just a little fling to start with, no strings attached. She’s someone I’ve known since my twenties, so there's this familiarity that comes with old friends rekindling their chemistry. It felt wrong, yes, because we've talked about getting married and settling down. But it also felt... necessary? Like, every once in a while, you just need to step outside the routine and do something spontaneous. Plus, she’s really supportive of my engagement now. We’re not going anywhere with this, but sometimes people say ‘I love you’ when they don’t even know each other well enough to have coffee together. Anyway, I'm sure it's all just a phase. Just wanted to get that off my chest and maybe remind myself why I’m here in the first place.