When I log on every couple of weeks, the majority of my read private messages are marked as "unread". There is no option to mark them as "read" which makes it hard to determine when I actually have a new Private Message.
] for an explanation of why your previously read PMs now appear as unread.
There is a way to mark them read, namely to read them. This would require you to open each and every message pretending to read it, until you have no unread messages left. I would however suggest that you delete old messages when you no longer need them; that will also prevent you from hitting the ceiling and being unable to receive new PMs.