Click on profile name > turn the phone to landscape mode > click "My Recent Activity". It's on the right side, under the badges. If you need to go back further than the most recent posts, the "View All" button is at the bottom of the list of posts.
Also, when I go to a message from an email, I sometimes can't get it to work at all. If I click on a forum notification, and then go to my profile, I can't get to my post history. This might just be specific to my phone and situation, but if I'm in mobile Gmail and click on a notification link to go to the forums, it sometimes won't change to landscape view properly. I have to choose to "Open in Chrome" from the browser menu, then it will work in landscape mode. That's on my Samsung Galaxy S8 using Gmail & Chrome. Might be super specific to my cell, though.