What should I do if I made a mistake in the last Tashahhud and spoke before the Taslim to end the prayer?

If the speech uttered was by mistake, then your prayer is valid if the speech was minimal. However, if it was extensive, then the prayer becomes void and must be repeated. The criterion for determining what constitutes minimal or extensive speech is based on common understanding. This applies if the speech occurred before the first tasleem. But if it occurred after the first tasleem, then your prayer is valid, according to the majority of scholars who affirm the validity of the prayer for one who confines oneself to a single tasleem.