While you are deciding whether or not to have it professionally restored, you can try a make-shift fix using aluminum foil. Simply lay the foil across the back of the mirror, shiney side facing the glass, or cut into small pieces to cover the bad spots. You may even find that, if you are using the mirror strictly for decorative purposes, you may not have to have it restored at all.
Before you get that done, you may wish to find out if what you have is an antique. Something that is worth money in the current condition could be "ruined" if redone. Or just leave it splotchy because you like it that way. Or ask that glass shop if they can put in a new mirror. Good Luck!
I know that a lot of "furniture refinishing shops" can resilver mirrors. If it is not important to you to keep the original mirror, you can have a new one cut for you at most glass shops for pretty cheap.
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