Leaving an abusing boyfriend, girlfriend or spouse is a particularly difficult task, and one which is often misunderstood. It is often obvious to someone on the outside of the relationship that abuse is happening, and this outside observer can find it difficult to understand, often laughable, how and why the abused stays with their partner.
Telling someone in an abusive relationship to 'get out' is not helpful. Often the abused partner will not have realised the abuse, or will feel it is their own fault. Pressure from another angle just makes the whole situation more overwhelming by increasing the feelings of failure and aloneness. It is this idea of it being the abused partner's fault that can make it so hard to leave an abusive relationship. The partner is often convinced it's his/her fault, so 'trying harder' will make it better. Being in a relationship means that at least, the partners loved once, and often love each other still. This can make it harder still to leave.
Even if the abused partner has realised the situation, fear can then prevent him/her from leaving. It is similar to the school bully-victim relationship, where, although the victim knows that they could go to a staff and be protected from the bully, the fact that the bully is the most immediate and prominent power over the victim means that the idea of someone else being able to do something is almost unbelieveable. This can be applied to the abused partner and abuser as victim and bully.
These, and many other factors - particularly involving children or finances - can make it hard to leave a relationship in which abuse occurs. Support from friends is vital, and can be the defining factor in the situation. Many couples separate, although it is hard, and both partners go on to lead normal lives. Sadly this is not always the case, but they can at least try.