My last appointment with T2 is this Thursday only she doesn't know it yet.

Should I contact her via email ahead of time to let her know?

or

Do I wait to say something at the very beginning of the appointment or do I wait till the end of the appointment to say I'm not going to be seeing her anymore?

I think it would be respectful to at least tell her at the beginning of the appointment so I can tell her why it's the last appointment etc... instead of going through the appointment and then telling her in the last 5 minutes.

What would you do or what have you done?