Ask yourself the tough questions
Start by confronting the emotions that have been building for awhile. What is your attitude toward your partner? Do you really love this person anymore? Is your relationship a positive force in your life? Prolonging what makes you and your partner feel bad is not only unhealthy, it can sometimes be cruel.
It's all too easy to be wishy-washy about a breakup. By making it official, you eliminate confusion and bad feelings down the line. Write a letter. In it, you might outline all the positive things that your relationship has been, what's been going on lately and why it has fizzled. Read the letter first yourself. Do the problems that you've listed seem fixable? If not, then it's time to deliver it. Sit down for a "meeting" in a neutral location. Keep the tone as upbeat as possible and don't place blame on yourself or your partner. Tell them that you're happy for what you had, but it's not working any more.
Make a plan for what will happen after the initial breakup. Taper contact slowly if you need to. It's likely that the longer the relationship, the longer it will take to truly separate. If you live together, consider moving out right away. See each other only when necessary, and then move your contact to phone, then to e-mail. If tapering is proving too painful, don't be afraid to cut things off cold turkey. Either way, the worst thing you can do once you've cut off or slowed down contact is to ramp it up again (even if that means ignoring birthdays, holidays or anniversaries entirely).
Put yourself in single mode
This doesn't mean jumping into a rebound relationship. Instead, rally your single friends around you, dig out your fun clothes, and go out. Meet new people of all kinds. Ask to be introduced to friends of friends to expand your social circle.
Focus on what makes you happy
Combat loneliness by taking up a new hobby, sport, or activity. Try new bars, restaurants, and stores. Focus on the activities that make you happy and make them a part of your weekly routine. The less you focus on your breakup, the more you'll focus on the potential for a happier life.