How do I find my old posts on this board.

You don't, because Riot says you don't want to.

http://boards.na.leagueoflegends.com/en/player/NA/EzraTwitch

You can find this in the future by clicking on your name, from any post, or your summoner icon in the box in the upper right.

Then you can filter by Comments or Discussions, if you'd like.