The figure of expression you are referring to is known as hyperbole. Hyperbole is a figure of speech in which exaggeration is used for emphasis or dramatic effect. In this case, when someone says "labour myself to death," they are using hyperbole to express the idea that they are working extremely hard or are overwhelmed by their workload. The phrase does not mean that they are literally going to die from work, but rather that they are emphasizing the intensity or heaviness of their labor.
