There should be a "show more" button for "participated in" as well. However, there's also a(nother) known bug which erases that participation history if a topic you participated in was deleted, so if that happens the history will only list topics you participated in after that happened.

"Topics I have participated in" shd hv a 'Show more' on its right side, clicking it will show all the other posts u made in chronological order backwards, u can keep clicking it till it doesn't appear anymore which shd indicate the 1st post u made ur post in right at the bottom.

Do note that apparently it seems the 'Show more' does not appear to all 4 some unknown reason according 2 some replies here, could be a bug but i'm not sure as i always see it.

When u hover ur mouse over any post, u'll notice a symbol & number appear to the right of the time stamp on top of that particular post. Clicking that number will bring u to the page which u can directly access that post so by bookmarking this page u can reach ur post easily in future.