Open Relationship Dynamics Explained (or Not?)

I recently ended a long-term relationship after four years together. It wasn't because we weren't compatible or happy; in fact, it was quite the opposite. We were in an open relationship for the past year, both of us feeling secure enough to explore other connections while maintaining our core bond. Here’s how it started: Last spring, my partner and I decided together that monogamy wasn’t working for either of us anymore. Neither of us wanted to break up or lose each other but felt a need for more emotional and sexual freedom. We agreed on rules and boundaries, nothing too strict, just mutual respect and communication. Everything seemed fine until two months ago when my partner started dating someone new regularly. This person was younger than both of us, and while they were respectful to me, there were moments where it felt like he prioritized her over our relationship. I didn’t mind that part so much, I'm okay with him dating whoever he wants, but the insecurity started when my own romantic encounters began to feel less exciting or special by comparison. Now I’m single again, and while part of me feels relieved to be out from under a dynamic that was straining at times, another part wonders if being open really worked for us. Did it help or hurt our relationship more than we knew? Should I have spoken up sooner about my feelings? I need advice, should I reach out to him again? Is this normal territory when you’re trying new things in an established relationship?