When you have been in a relationship for so long, suddenly breaking up will be devastatingly hurtful and many times when people find themselves in situations like this, they often find themselves returning to their partners even when they know they probably should not.
The familiarity of a relationship is something that you cannot forget easily, especially if it is one that lasted for a while and so many great memories were created while you were at.
Even the relationship that ended for terrible, unbearable reasons such as remorseless cheating or domestic violence, are usually never filled with only bad times. It could be infrequent, but there will be highs as well. It is very unlikely that a relationship will be filled with only bad times.