How to get out of a long relationship without hurting anyone? :-D

You can't do this without causing pain. Give up that idea because it's just not possible.

It's like taking off a bandaid. Faster is better. Delaying it only makes it worse. Have enough respect for her to assume she can, eventually, find someone who can appreciate her as you cannot.

Pick a place that is not her home or yours. Let it be a place with a little privacy, but also has some people around. A park is good. Or a private booth in an uncrowded restaurant.

Tell her: "You are the kind of woman who can make some man very happy. But I have figured out that I am [i]not that man.[/i]It's over, Zelda. I will always wish you the best."

Expect arguments, tears, maybe even a major scene. Do not say anything. Do not argue with her or answer her. Just listen or observe. Give it a set amount of time. 3-5 minutes is good. When she has vented for a few minutes, wait until she stops or pauses and then say:

"Zelda, I can see how strongly you feel about this. Nevertheless, it's [i]over."[/i]

If she keeps on crying, arguing, guilt tripping you, get up and say:
" I never meant to hurt you. I see a great future ahead for you...but not with me. Goodbye, Zelda."

Leave and do not contact her. If you are concerned or worried because you fear she might hurt herself (or she has threatened to do so) then phone a friend or relative of hers to come and help as you leave or right afterward. If she has no one to help, do not stick around and try to act as her therapist. Call a local suicide hotline or other emergency service.

End contact. Remember the bandaid analogy. Most people are stronger than we think (or they think). Give her a chance to find somebody who will appreciate her.