I sent a message to someone that I didn't mean to, although they haven't seen it yet. It was very personal and I don't want them to read it. Is this possible to accomplish?
How to successfully delete a Facebook message that you sent someone from their inbox remotely
You can't delete message from someone else inbox, it has seen or not that doesn't matter.
There is no way of deleting your message from your friend's Chat Inbox or from a Conversation once it is sent. Messages will only be deleted from your Chat Inbox or Conversation.
The official article from Facebook Help Center:
How do I delete messages or a conversation?
You can delete messages one at a time, or you can delete an entire conversation.
Note: Once you choose to delete a conversation, this can't be undone.
Keep in mind that deleting a message or conversation from your inbox won't delete it from your friend's inbox. It isn't possible to delete sent or received messages from a friend's inbox.