My own posts

You, like many, have encountered an ongoing issue: what would be called "My Activity" anyplace else is called "My Subscriptions." In addition to the method that lujaire46 has posted, you can access your "subscription" directly by clicking your avatar in the upper right corner of any forum page:

Hey @tractorgirl, don’t worry…you’re not overlooking a quick link.

You can find your recent activity by navigating to your subscriptions tab on your profile and filtering by participated/authored. For posts that predate that, try googling ‘Apple discussions’ + your username.