Finding a past topic

Just wondering how I can search a past topic in the forums?

Actually it's pretty easy once you know how, just not obvious.

Go to the Support area - you can use the button in the menu stripe at the top of the forums. That offers a large Search bar, that you can use for searching.

Yes, you're searching the Support docs - but as it happens, it's also searching the forums too, and any matching forum post will be returned in the result. :-)

Is there a link to show me all of the topics that I've posted in myself?

No, but there should be. I could've sworn there was a page to see your posts but I can't find one -- noting it down as a todo :-)