Well, to a certain extent it is understandable to feel low and depressed at the breakup of a long relationship.This can well be contributed to the extent of dependence on the relationship and the extent and quality of bonding, besides the suddenness of breakup.In fact break up of a relationship can lead to reactions typical of loss: loss of self esteem, loss of self regard / self worth, loss of a person to relate to / talk to, and can leave one wondering about why did it happen to them etc and can further lead to grief.The key here is if these feelings are increasing in severity over time and start affecting one's day to day functioning - one should take extra active efforts to seek help: like spending time talking about their feelings and the strength of their relationship to a Psychologist, keep busy in activities that they usually like through the day, and if things are unbearable consider visiting a psychiatrist for short term medications.These reactions are quite common and easily manageable.