You can use the topic-filter to filter for topics you created in a category. For example, https://meta.discourse.org/filter?q=category%3Asupport%20created-by%3AHelga_Razinkova%20.
Or you search for posts within a category (#categoryname) written by you (@username) and limit results to first posts in topics (in:first). For example https://meta.discourse.org/search?q=%23support%20%40Helga_Razinkova%20in%3Afirst