If your relationship gives you more misery than pleasure and more pain than fun, sit down with a pencil and paper and figure out what you want and what you're willing to offer to get it. If you discover that there is nothing the other person can give (or anything you're willing to relinquish), then that should tell you something. Similarly, if you're in a relationship that used to work but has now turned rancid because one of you has moved or changed or cheated, you can't go back. However, you can evaluate if there is anything your partner can offer that has value to you and anything that you're willing to offer to get it. If so, get busy and figure it out and offer it. If not, it's time to do that grown-up thing and break up without bloodshed or nastiness.
Delivering the news
The relationship isn't working for you. It really is the end. Breaking up is as important a skill as any other part of dating. It's not fair to just disappear without a word. The world's too small a place, and you're too big a person, so don't even think about it. Now the goal is to end it with the minimum blood loss, nastiness, and pain. When you finally decide to make the break, how do you actually go about doing it?
The first temptation to be avoided is the need to blame somebody or something. Because there are only the two of you, it's logical that you will decide, heroically of course, to make it all your fault, even though you know it's not true: "You're too good for someone like me," "I don't deserve you," — both of which mean you want out now. Or you could blame your partner: "You never loved me enough," "You cared more about your work than you did me," "You've never really gotten over your first love," "You've put on weight, lost hair, gotten moody..." Yada, yada, yada.
You don't need to fall into either trap. All you have to do to be dignified is to be specific about your feelings without laying blame. It doesn't matter in the long run whose fault it is, and avoiding blame spares you both a lot of pain.
To avoid the blame game, try saying, "I feel ..." rather than "You are ..." and no, it isn't okay to say, "I feel you're a rat." This approach is okay only if you follow up with something about yourself, like, "I feel neglected when you work weekend after weekend." (Of course, if you had been able to say this when you were feeling it, the relationship might not be beyond redemption at this point.) If you're specific now, at least both of you can look at the data as dispassionately as possible rather than feeling that either of you failed.
Don't ask why
When a relationship is over, the "why" is less relevant than the "how" — how are we both going to walk away and be able to live our lives without scars or regrets? Sometimes, a perfectly good relationship is a perfectly good relationship only for a while. That doesn't mean it was bad, only that it wasn't long-term.
If the two of you are specific, you'll know what went wrong and what, perhaps, either of you could do differently next time. The why may be lost in the mists of time or be a proper subject for therapy, but when you're going your separate ways, getting stuck in the past feels incredibly painful. And the why is in the past, often clouded and sometimes unknowable. When you're reduced to asking why, you're both sunk, and there are no comforting reasons to be had.