For the Desktop version of Facebook go to:
https://www.facebook.com/[yourprofilename]/allactivity
Or click in the top right menu and choose Activity Log:
On the next page, you’ll see the following:
At the top right from where it says “Activity Feed” is a smaller search field. This will allow you to search any previous post that you’ve made on Facebook in the past.
This is a great way to find old posts quickly.