Once you write something down and send it to someone they can choose to keep it forever. There's no way to enforce ephemerality.

The proper solution to people holding you to something you may have said at some point during your growth that you no longer agree with is to tell them you've changed your mind. If they still try to hold you to it tell them to go fuck themselves. That's the only way.

If it's something you don't want anyone but the recipient to know, make sure you trust them to protect and/or destroy the message and use a secure channel. That's all you can do.