How can I search my Topics in Steamtrades?

Look at the top bar.
See that thing on the top right? That's the search bar. Just type in your topic header and you should find your topic.

i know that feeling
either try to remember what your title was or use your history.
if all else fail, go back and look for it.

site:steamtrades.com FrostFox1998

Copy that into Google.

Here is one of them.