The ability to edit and delete posts is limited for a reason. If users were free to remove their posts whenever they wanted, we’d end up with broken discussions, where many posts would be missing and the replies wouldn’t make sense. In a way, a forum is similar to the old system of mailing lists, where you sent messages to a server and couldn’t take them back.
I understand your situation, but unfortunately, the platform doesn’t distinguish between public and private topics. It applies the same limits to them. This is why some users use the workaround of flagging their posts, and a moderator will remove them. Others opt for alternatives like private note services.
Btw you can also use chat DMs which are only temporary.