I think in addition to saying "I have other things I need to discuss today, and I prefer that we go on to this subject _x_" you might want to keep something as a backup. If your T continues to pursue the subject you don't want to discuss as that moment, I think an option is to suggest that you could discuss it at another session if your T still considers it important, but right now, you think it is very important to talk about the topic that is on your mind and heart today, and you want to focus on that for the session. (Sometimes I've had this happen, too, and I've had to come up with a gameplan for dealing with it!)

Wishing you well,
ErinBear