The forum will only keep copies of any messages you send if you check "Save a copy in my outbox" under your message box manually or set it as checked by default in your profile ("Personal Messaging" / Setting "Save a copy of each personal message in my sent items by default").
When clicking on "My Messages" on the top bar, an option "Sent Items" will show on the menu on the left. If you click it, it will show you any stored sent messages.
If you have the contacts of the people you replied to, you may ask them to send you back a copy of your previous messages. Hope this helps.