Generally what happens is that as new discussions are posted, the old ones kind of get buried towards the bottom. This happens really fast, because Eden is a BIG community.
You can find your threads in a few different ways. Across the top of the forums, you'll see these 4 tabs, labeled "All Discussions", "Favorite Discussions", "Favorite Contributors" and "My Discussions and Posts".
All the threads you've started and posted in will be found by clicking on the "My discussions and posts" tab.
You can also "Favorite" threads, by clicking the little heart next to the thread title (it will turn orange). All your favorites will appear in the "Favorite discussions" tab. You can also "favorite" contributors by clicking the little heart next to their name (it will turn green). Then you can find all threads they've posted in and started on the "Favorite contributors" tab.
Using those options, you can "favorite" any thread you start, and then find it quickly under your "Favorite discussions".
Hope this helps!