Go to your profile, on the right hand side will be a menu, which by default will be set to "My Content". Below that is a "My Activity" tab. Click that and a list of your past actions will be shown. A double horizontal bar symbol will be next to forum activity.
You could also go to your My Account page. At the top will be a double menu. Click the "Following" tab on the top menu and then on the second menu the "Forums You Follow" tab. By default you should be following any forums that you have created or participated in, so it should be in that list.