If you click on UserCP, then every thread you posted in, you should be automactically "subscribed" to... unless it's turned off.
I have mine turned off cause I post too much!
To see if your "subscriptions" is turned on, go to UserCP >> Settings and Options >> Edit Options >> and scroll about a quarter of the way to "Default Thread Subscription Mode" >> change the dropdown >> then click SAVE CHANGES at the bottom of the page.