As with all magic, if the spell is strong enough, the mirror will be unable to reflect the spell, and will either shatter after absorbing the spell or simply break apart. Given that your magic feeds off other magic, the threshold would be lower for you I imagine.

The real question is, how do reflected attacks work? Can you drain yourself of magic with your reflected attacks? If so, where does the magic go? To Illedrith? Back to yourself? Questions.