If you view your profile (accessible by clicking on the portrait dropdown in the header and clicking "profile" at the top of the list) and click "see my activity" (towards the upper right) you'll get a list of all your activity on the site. There's a list of different site areas/content types down the left hand side of the page that you can use to filter for different types of content - if you scroll down to the "forums" heading and choose "topics" under that, you should get a list of all the topics you've started. Choosing "posts" instead would include all of the topics you've replied to as well.
Note that this is a list you can browse through rather than an actual search; if it's reasonably recent or you haven't created too many topics (your profile is relatively new and has less than one page of results ) it's pretty quick and easy to use though.
Alternatively, there should be a "content I started" option under the "search" heading from the top navigation bar. This will give you a similar content list which you can filter with the options along the top - the second option for "content types" has an option for "topics" which would get you forum topics. This list is (by default) ordered by most recent activity, including replies from other members.
Or, another alternative. on the search page there's a blue "+ search by author" option you can use to restrict search results to content created by a certain member (such as yourself). There are various options on the page to filter different content types, date ranges, etc.
Hopefully, one of those options does what you want, hope that helps. It's a bit confusing with the current platform, but there are definitely plenty of options - we'll be trying to make it a bit more straightforward with the next version of the site.
//EDIT: I described all of the above options as viewed on desktop. All the functionality should still be there on mobile but might look a little different.