A Slip Into Old Patterns

I’ve been in my current relationship for almost two years now, and it’s genuinely good, supportive, intimate, loving. Yet recently, I found myself slipping into old patterns from before we met, when I was still navigating the complexities of being single after a long-term breakup. One night, out of nowhere, I texted an ex-boyfriend who's in a committed relationship himself. We had no intention of acting on it and didn’t even talk about anything specific, just an acknowledgment that we were there for each other if needed. It feels like a betrayal to my partner but also like a defense mechanism from when I felt more vulnerable. Why did this happen, and how do I reconcile these conflicting feelings?