Such things only have power if you believe in them.
If you do, though, and wish to remedy the problem using the ancient methods those who originally came up with the superstition used, here you go:
"An ancient myth our ancestors believed was that the image in a mirror is our actual soul. A broken mirror represented the soul being astray from your body. To break the spell of misfortune, you must wait seven hours (one for each year of bad luck) before picking up the broken pieces, and bury them outside in the moonlight."
I also found:
"Mirrors are the perfect example of an everyday household furnishing fraught with
superstition. Probably the most common belief concerning mirrors is that their
breakage causes seven years of bad luck. Whatever the origins of this belief,
here are some methods to counteract the curse.
* Throw salt over your shoulder.
* Directly after breaking the mirror, turn around three times counterclockwise.
* Burn the mirror, or at least blacken its shards in the flames of a fire. Save
the fragments for a year and then bury them, and the curse will be counteracted
(so you've only had to live with one year of bad luck).
* On the first night after breaking the mirror, light seven white candles and
blow them out at midnight in one breath.
* Touch a tombstone with a piece of the mirror and the hex will be lifted.
Any of these rites can be used, but bear in mind that the "curse" of the broken mirror is usually a self-created one."
Best of luck!