Prepare yourself, even when you know that breaking up with them will be the best for you, they will fight the decision tooth and nail. They will be worried about how it will make them look and they will have to find another person to abuse, which will take time and effort for them.
Decide that you will not allow their manipulation. They will suddenly soften to you, they will seem sweet and they may even claim that they will change for you. But for narcissists, real change comes over time with professional help. It doesn't just happen overnight because you're afraid of losing a relationship.
Make a clean break. Get them their stuff back from your place before you break up or on the day of. Narcissists will try to talk their way back in your door claiming unfinished business or the need to talk. Translation? They are going to try to get you back into the relationship.
If you know you will be vulnerable to their advances, put preventative measures in place to end the contact before it even begins. Change your phone number, get them their posessions back, find some new hangouts, and reconnect with your own friends. It's not cruel, it's healthy! Break ups don't have to be long, drawn out, soap opera dramas! In fact, you break up to end the drama.
Protect yourself emotionally. Narcissists will come back like a boomerang. Even after they accept the break-up, they will show up with gifts at your door. They will want to talk fondly of the relationship. They will want to control what you say to others about why you broke up. They want control and they want to come out looking like a hero. Don't participate in this weird ritual.
Grieve the relationship. Cry, get angry, deal with your emotions, get counseling, or read some books on dealing with the after-effects of a relationship like this. It can be more complex than breaking up with an emotionally stable person, and you may find that you start to realize that you were being emotionally abused.
Process what just happened. This is a lot to comprehend, if you don't let it out and try to deal with it, you could repeat this pattern and find yourself in a relationship with another narcissist. Take the time to learn about yourself in order to truly move on and be happy in romantic relationships in the future. You deserve it!