Are the seven years of bad luck after breaking a mirror a superstition or is it a true thing?
The belief that breaking a mirror will bring seven years of bad luck is a superstition. There is no scientific evidence or historical fact to support the idea that breaking a mirror will cause a person to experience seven years of bad luck. 

Superstitions like this one have been passed down through generations and are often based on old beliefs and folklore. They are not considered true in a scientific or rational sense.
How would I go about proving/disproving the claim about breaking a mirror?
As the belief of seven years of bad luck after breaking a mirror is a superstition, it cannot be scientifically proven or disproven. Superstitions are cultural beliefs that are passed down through generations but are not based on empirical evidence.

However, you can try to test the hypothesis by breaking a mirror and observing if you experience any instances of bad luck for seven years afterwards. Keep in mind, however, that such an experiment would not be scientifically valid, as it is subject to various biases and confounding factors that could influence the outcome.

In general, it is important to be cautious of making causal claims based on superstitions and to base our beliefs and decisions on evidence-based practices and scientific knowledge.