See the past as a dream.
What I've experienced, and know others have too, is that after being with a girl for a certain amount of time, the relationship just isn't the same as it used to be. After being someone with so long, it's know longer knew which often makes a relationship so great. Fortunately, Buddhism has answers to these problems.
One of the teachings of the Buddha that inspires me so much is that partners should serve as each others spiritual mentors and teachers. To me that means that partners ought to act as spiritual role models whether they are Christian, Buddhist or atheist.
Buddhism also teaches about how the world is illusory. Sometimes when we've been with a partner for so long, another person tends to come our way and we think I'm in love with this person. despite already having a relationship. You might find yourself torn, thinking I've been with my partner for so long, but this person brings out something in me I never new.
Fortunately, as Buddhism teaches the world is illusory, we can realize that this new person represents in us a part of ourselves we are not currently aware of. It may be helpful to explore ourselves. We can often find new parts of ourselves when we visit another country, help out a needy child or take a walk in an unfamiliar part of town.
The Buddhist teaching is the Law of Karma, that everything we do leaves an imprint on ourselves. We are often, a humans, "in love with the idea of being in love." I've felt before that I've wanted to feel in love again, for the first time. Fortunately, that you that fell in love is still there hidden inside you. You may recapture the imprint, in which memories survive, through making a love journal of all your past experiences with your lover or going to a place with special significance to your relationship.
Finally, as the Buddha said, the past should be viewed as a dream. From that point of view, one's relationship aught not to matter how old it is, because that memory, though important, is like a dream. Identification the ego can be loosened by practices that cultivate compassion.
My favorite is the Mantra of Chenrizig, the Bodhisattva of Compassion, who Tibetan Buddhists believe the Dalai Lama is a reincarnation of. Often chanted one hundred eight times for the number of beads on a Buddhist prayer bead mala, it goes:
Om Mani Padme Hum
This translates roughly as "Hail, Jewel in the Lotus."
The next chant that I particularly like is the Metta chant. It goes something like this:
May all beings be happy.