Research by anthropologist Helen Fisher, has shown that intense romantic feelings tend to wane after a relationship's early stages settle into calmer, less exciting stages of attachment (Reference 1, page 92; Reference 2). As a relationship's initial fire wanes, it can be hard to tell whether it's nearing the end or has simply hit a rut. If the relationship is meaningful to both you and your partner, then it's worth it to do all you can to try reigniting your fire before giving up and parting ways.
Evaluate why you're unhappy. Assess specific issues that could derail your relationship if left unaddressed.
Lay ground rules for a calm discussion with your partner. Agree that you will not argue, but focus on attacking your problems rather than one another. Focus on hearing one another and expressing yourselves fully and earnestly.
Take time away -- hours, a day, even a week if needed -- to consider realistic concessions or resolutions to propose, once everything is out in the open. Evaluate to what degree you must change your own actions or be more realistic about your wishes. Be honest with yourself about your needs, what you can realistically offer, and what compromises you can accept. Reconvene to exchange your thoughts and work toward resolution once you've both determined what seems feasible and necessary respectively.
Seek outside counselling or a mediator if you can't agree on remedies. Couples therapy is not just for married people, and may help you understand one anothers perspectives, what seems fair and why recommended measures are necessary.
Reignite your fire by regularly engaging in exciting activities together like roller coaster rides, horror movies, mountain climbing and other adventurous engagements. Arthur Aron, psychology professor at Stony Brook University, conducted a 2000 study which determined that such novel activities help revive romance and boost relationship quality. High levels of neurotransmitters dopamine, adrenalin and norepinephrine, which contribute to thrilling feelings of early romance, are also stimulated in stressful, risky and frightening situations -- and your brain won't know the difference between one and the other (References 3, 4).
Recreate Aron's 1991 experiment, which demonstrated the power of eye contact and personal disclosure in establishing deep connections. He sat two strangers in a room together for 90 minutes and instructed them to exchange intimate information like most embarrassing moments. At intervals, a researcher prompted them to stare into one another's eyes silently for two minutes, and express what they already liked about one another. Despite being instructed to leave through separate doors so as not to feel pressured to spend more time together, his first two subjects married one another six months after the experiment. Because self-disclosure feels risky and leaves individuals feeling vulnerable to one another, it fosters trust and deepens connection. Recreate the experiment literally, or take your cue from its premise and commit to being more open with one another henceforth (Reference 5).
Move on if it becomes clear the relationship really isn't going to work. If effort feels one-sided, you won't be happy in the long run.