can't get into encrypted .dmg files even though I could the day before and I am 100% certain the password is correct
Without backups, all you can do is try variations on your password.  The encryption system would be useless if it had a backdoor to circumvent your password. 


  The password is absolutely, positively correct


I suspect that when you setup the DMG what you actually typed in the password field was not exactly what you thought you typed - and you repeated the error. It is possible to consistently type the wrong password - were you typing very quickly, or were you tired, for example?

Try opening TextEdit and typing in your password many times, see what kind of errors you make when typing quickly,  pressing/not pressing shift, variations with Caps Lock set, accidentally pressing two keys instead of one, vary how you were sitting with respect to the keyboard, etc. You can then copy each password variant from TextEdit into the DMG dialog password field to try to open the DMG. Good luck.