Break-ups feel bad for a very good reason: it is in our nature to form attachment bonds with our partners - bonds that resemble in intensity those we made with our parents. When those bonds break, it hurts.
One particular kind of breakup is the subject of this post: the breakup in which one person wants to end the relationship, but the other person doesn't. This kind of break-up has a special painfulness to it, because one person gets their heart broken and the other has to live with hurting someone they really care about, in addition to being in pain from the separation themselves.
Breakups raise primal negative feelings: guilt, sadness, anger, and fear. It can be hard anyone to experience these feelings without wanting to stop them or control them or manage them in some way. When people try to manage their emotions, one of the first things they do is try to figure out the cause of these feelings so they can predict or control them in the future. The person who is being broken up with may either blame themselves ("what did I do wrong?") or they blame the partner who broke up with them ("how could you hurt me like this?"). Typically, both types of blame are attempts to escape from the real issue, which is how much anger, hurt, or despair one is currently feeling. Typically, these types of blame are also distortions of the truth.
Blaming oneself is a distortion. It is worth remembering that relationships' longevity and happiness are about the dynamic or fit between two people, not about the inherent worth of either party. Our culture glorifies relationships and many people assume that being in a relationship is a sign of desirability or worth, and conversely, being single is a sign of undesirability and worthlessness. In my experience, being in a relationship or being married have very little to do with either desirability or worth.
Blaming one's partner is a distortion. Couples therapist John Gottman has observed that in every relationship disagreement, there are two valid points of view, not just one. This truth goes even broader. Indeed, every relationship is really two relationships. In other words, the relationship can feel very different to one partner than it feels to the other. This means that the relationship that one person was really excited about ("how could you break up with me? Things were so good!"), did not necessarily feel exciting to the other person. Often, the person doing the breaking up felt fundamentally dissatisfied with the relationship. The truth is, if you are the person who is being broken up with, you deserve someone who will love you without being fundamentally dissatisfied with what you offer. The person who broke up with you cannot fill that role, and by breaking up with you, they are freeing you to find someone who can.