Adult relationships are nothing like what the movies promised us. They are not filled with dramatic romantic gestures on the daily, and most shocking of all, that crazy-in-love feeling doesn't last forever. Real love has lulls; it's jam-packed with mundane moments. But just because those heart palpitations you regularly experienced when you first met have considerably slowed, doesn't mean your relationship has lost its luster.
A comfortable love is not necessarily a sign of an impending break-up. Instead of gauging your feelings based on excitement (an immature notion), base it on whether you still actually like the person. Being "in love" is a feeling that comes and goes in waves, but you should always like the person you're with.
If you still care about her, but find yourself irritated with every thing she does—from her "quirky" habit of twirling her hair while she talks, to larger personality traits, like the fact that she's not as committed to fitness as you are, or has no interest in culture (or even leaving your apartment)—you may be on your way to parting ways.