Only one answer here - buy a new mirror the correct size.
You will not be able to cut down the oversized murror you have without causing damage to the reflective surface. It is the quality of the finely worked surface and finish that we pay for.
Sorry for the bad news.
Oh, except if you are capable of refiguring the damaged surface to restore it to its original sub-wavelength condition and re-aluminize to the same finish, be it enhanced aluminium or protected aluminium. There are a couple of businesses that can do the re-aluminizing for you.
Alex.
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