1) unsubscribe
--menu bar up above
-click on quick links
-click on subscribed threads.
up they'll pop
-on a given subscription, click on the box at the far right to select it (or more than one for multiple threads at the same time).
-drop down to the bottom of that list, lower right is 'subscribed threads' box. click that, select 'delete subscription', click 'go'.
2) find previous posts you've made / see if you're in a thread.
on any list of threads, whether under general, new threads, what's new, your subscription list, wherever; if you've made a post in that thread there is a little green arrow on the letter icon on the left of that thread title. hover your mouse over it and it will even tell you the date/time of the most recent post you've made in that thread.
more complicated version: click on your name top right of the page, click on 'recent posts' on the tab to the left side of the screen on that page, and you can look through/access all the posts you've made from most recent to oldest.