Hypothetical question: Ending a relationship.. what is best?

Suppose you are miserable in a relationship and have been for a long time. You are certain that your significant other is MUCH better off without you (i.e. takes care of themselves and improves when you're not around.) However, you're very much attached and your significant other is VERY dependent on you. You are afraid they will deteriorate emotionally and possibly commit suicide if you end the relationship, but you know deep down that you have to. How would you go about doing it, in a way that would result in the least amount of pain and heartache?