I sometimes find it almost impossible to find out where I posted something. It really does annoy me I have to say and more often that not I give up looking, like today.
Souxi, there are several ways to find a thread where you've posted in. We have an "Advanced Search" function, where you can search for keyword(s) and additionally by which poster the post is you're looking for. It's in the left yellow box on top of all BM pages.
Additionally you can check all your previous posts by going to your profile and click "Show last posts of this person" at the bottom of your profile page. Of course this would be quite time consuming if the post you're looking for is from months back. But if you're looking for a post from, let's say the last week, you'll find it quickly.
And there's the "Show new replies to your posts" function, which is also on the top of every BetterMost page, in the left yellow box. This will show all threads you've posted in and where new replies have been posted since your last visit.
Hope this helps.