Accepting the fact that you are both human.

When two people get into a mutually intense romantic love relationship, they tend to idealize each other. There is also the rush of being the apple of somebody's eye. As time passes and the intensity fades, it can be hard to accept that you no longer garner this person's every last bit of attention. We come to the realization that this person finds other people attractive, funny, intriguing, and engaging. This can drive someone mad. It is usually at this point in the relationship where insecurities poison the relationship and couples resort to fighting to bring back the intense feelings that have passed over time, but this is only sustainable for so long. Accepting the fact that you are both human is the only way to avoid this. Look at yourself first-- Do you find other people attractive, funny, intriguing, and engaging? Unless you live under a rock, I'm sure you do. By accepting this in yourself, you must accept it in her. The common fallacy is that these feelings invalidate a loving relationship. Suppressing these feeling can lead to resentment and ultimately perversion (which I do not mean in the commonly interpreted sense.)

If you really love someone, you must concede these unrealistic, greedy expectations of each other. To say "we aren't biologically designed to love someone for that long" simply isn't true, you're just throwing your hands up in the air and giving up. It is a statement conjured up by your own insecurities.