search the forums for various threads on "application database full" and the likes, its probably not your internal space thats full and you likely just need some temporary files removed, there are numerous posts, the easyest being the deletion of several archives and log files.
heres a snippet of what i do first, using InternalZ i do the following.
and delete ALL messages.blahblah, ie, messages, messages.0.gz, messages.1.gz etc
and delete filenotify.db and downloadhistory.db
delete the whole folder, then re-create the attachment folder with the exact name, the subfolders inside you may have seen before wiping it will just auto recreate when required.
FULL device reboot after their all deleted, some reported they needed more than 1.
also note if this doesnt work there is a post where you need to do a bit more tekkie stuff to make the issue go away or at least slow it down a hell of a lot.
Workaround for Application Database is Full error | webOS Nation
read it totally, see if its something you can do, and proceed if you dont mind making a hash of things and potentially doing something bad (shouldnt be an issue if done 100% correctly, just dont rush), if you have no problem with it go right ahead, if your not confident maybe just try removing some of the stuff i mentioned above and hoping that works for a while, until a better more automated method becomes available.