Nope, it's not possible.
When you already hit 300 good posts reported, you can only view and track your past 30 days reports to see the status is unhandled, good, or bad.
I see no problem if the forum will want to implement to withdraw reported post, but if you mistakenly reported your post e.g. duplicate or high quality post, the moderators will not likely to proceed your report and maybe it will stay in unhandled.
Such mistake like reporting a post unintentionally will continue happening, Cuz This sometimes may occur as a result of faulty devices like phones, laptops, which are used in carrying out this process. If such is done to someone else post, be it intentional or unintentional, it calls for the implication of that person's account. So the best to amend this is to withdraw the report. I think it will be wise if the forum can implement withdrawal of reported post.