Click on the blue circle on your Forum profile to find all your content and followed posts.
When you go to your Vivaldi Forum profile on https://forum.vivaldi.net/me (or click on your profile image in the top right corner of the Forum and then on your username), you’ll find a 🔵 blue circle on the right side of the profile page.
Clicking on the button will open a menu with a lot of useful options. For example, you can see a list of all the discussions you’ve started (Topics) and comments you’ve made (Posts). Useful, when you’re looking for that one post you made ages ago.