Google is your friend?
Many available password 'crackers' for Excel workbooks and worksheets, some
free, some not.  Some not-free ones permit a download of demo that will
provide something like first X number of characters or every other character
of a password or limit the length of the password to be detected.  But those
might give you hint enough to jog your memory.

Typically such crackers don't even tell you the original password, but they
will give you a word that will work to unlock it.  Then you just protect it
again using a word you do want to use.

Search for phrases like
Excel password detector
or
Excel password recovery

Make a backup copy of your file before attempting any password recovery on
it just in case.

If bad goes to worse and you can't seem to get it unlocked and are willing
to entrust me with the file, email it as an attachment to
HelpFrom@jlathamsite.com and I'll use one of the commercial products that I
own to unlock it and return it to you without the protection so you can
re-apply a more memorable password to it.

See

   http://www.mcgimpsey.com/excel/removepwords.html

or

   http://www.mcgimpsey.com/excel/fileandvbapwords.html