Notepad++ has no notion of “post”. It just sees a big blob of text, and tries to apply the search string you give it. If you search for just your username, that’s the only piece that will match. If you use a more complicated regular expression (“regex”), you can define a fancy search that will match more.

If you gave more details about what a “post” is formatted in text, we could help you come up with the right regex. For example, if the post is username: blah blah blah [newline][newline], then something like ^username: .*?\R\R would find everything from username: to the two newlines in a row.

But Notepad++ is not just going to “hide” everything else, and allow you to look at just that. As you do each Find Next, it will just highlight the entire match. It might be possible, if you gave us examples of what this file would look like, to be able to strip out “posts” that aren’t yours, instead. (Similar to the above, but matching anything but your username at the start of the post). But until you give us example data that matches your formatting, and show which posts you’d like to keep and which to delete, there is no way we could randomly come up with the right regex to help you.

Click the </> button in your reply. Put some data in the code-box that creates. Make sure the data matches your actual data, and is formatted exactly the same. Indicate which “posts” you’d like to keep vs delete. Make sure you give us a description of how you, as a human, would recognize where one post starts and ends.

Update: it might actually be easier to use the Mark dialog to mark all the username posts, rather than the find/replace to delete the non-username posts. That would allow “copy marked lines”, and then the username-only ones could be pasted in a new file, rather than deleting the non-username posts from the original. But it depends on the shape of the data, and whether you’d prefer to just delete from your old file or create a new one…