Sometimes breaking a mirror means you can't go back - it's like the tarot card The Tower - meaning, the past is in the past, dust, a skeleton. The mirror can't be fixed. Maybe you can think of it that way?
In the old days, glass/mirror material was very expensive and precious, so of course, breaking it back then was pretty serious-so hence the 7 years bad luck? I don't think it applies today... :D
The Romans believed breaking a mirror took part of your soul? I know in some First Nation cultures, even looking in a mirror takes part of your soul...
Since the mirror is dead, I'd say, throw it out. Start new!