You've misunderstood how things work, I think. A thread, once started, is here permanently (you now have two on the same subject), but by default only those which have been added to in the last day or so are displayed. Simply set the display filter on the main page to a longer time-span and you can see all the threads added to in the last week, for example. If you can't find a thread you have started, just click on your name at the head of a post (you'll see that it's a hyperlink), and you'll get a list of everything you've ever posted here under that name. More details in the FAQ, which you will find very informative.