It is a configurable setting, we currently have the forums set to only allow edits to your post for the first 24 hours after you first post it - after which time it is locked and only moderators can edit it.
If you have a problem with one of your posts - notify the moderators and they can fix it if necessary. To make it easier you can use the "report this post" option (red and white triangle with a vertical black line in it) to notify the moderators.