how can i specify the index of a list variable when the index is stored on another variable?

my variable is %password% and my index i have it stored in %counter%

i tried %password[%counter%]% but gives me a sysntax error

what am trying to do is a script that changes my passwords automatically at noon every day, passwords are store in a txt file and i store the index(las password i used) in text file called counter.txt,, so when the job is trigered the first thing i do is assing the contents of the password textfile to %password% and index.txt to %counter%

but how do i retrieve the new password?