I wish to propose a form to allow a contact to unsubscribe from a newletter but not from the set to which it subscribes.
currently, the form I created unsubscribed from all newsletters.
It is possible? If yes, how ?
You should be able to do this by directing folk to eg www.yourdomain.org/civicrm/mailing/subscribe and if you have your Mailing List groups set up correctly (ie as public) then they should be able to unsub.
for folk using Drupal you can also do this via Webform
also review instructions at https://docs.civicrm.org/user/en/latest/email/set-up/