Here is the description of how to achieve regular status:
You have to maintain a fairly constant level of visiting, posting, giving/receiving likes, and reading posts/threads over a rolling 100 day period. I don't know if there is any way to actually track which hurdle tripped you up, though. You might be able to use the "users" (access it by clicking the three horizontal lines between your user icon and the search button) function to get a sense of your activity level, but the date options they give you there don't line up with the 100 day regular requirement, so that probably won't help. I checked the list of people who have the regular badge, but it only lists people that currently have it, not those that ever earned it, so that doesn't help either.
Edit to add: I found your name in the list of regular custom titles that Actreal maintains, so you definitely had regular status before.