I've been a user of Facebook since 2006 and I need change the privacy setting of all my posts to "only me" so no one can see them.
Is this possible? How can I achieve this?
It doesn't appear that you can.
The "Limit Past Posts" tool in the Privacy Settings and Tools section allows you to bulk change any posts that were shared with "Public" or "Friends of Friends" to be shared with just "Friends". There is no option to change it to any other setting. (See Facebook Help) There is no other tool built into Facebook to do this sort of thing.
Your options would then appear to be
Or, delete your Facebook account.