The relationship isn’t boring. Life is boring, and you’re just now noticing for the first time as an adult because you aren’t being distracted by some youthful flavor of chaos.
If you want to keep from becoming ungrateful, do some serious soul-searching and identify exactly what’s missing in your life now that you’re in a stable, healthy relationship. “Things aren’t fresh anymore” isn’t a good enough answer. Be very fucking specific, and very fucking honest with yourself. Remember, fresh isn’t the same thing as exciting, and excitement isn’t the same thing as chaos.
If you can’t come up with a specific and valid problem other than the mild state of boredom known as everyday life, then guess what? This is as good as it gets. Take that as good news or bad. Either way, you’d be wise not to go searching for a taste of chaos. That shit isn’t a spice.