why must i have to keep resetting passwords?

about once a month i am told my password is wrong, did i forget it? no i didn't forget it, i have my passwords written down on a rolodex. nobody messes with my rolodex. and when i go to reset i type the correct password in am told that i must use a pw i haven't used in the last year! this has gotten very maddening. what is going on? why does this keep happening?