If it becomes too much, I take a break. I don't leave the room (well, I've done that two or three times too), I just ask (literal translation is 'for a minute, not?' meaning: can we not talk for a minute?. Or sometimes I hold up my hand), usually turn my head away, and then we sit in silence until I'm ready to continue. Usually no more than about 2 minutes. If they talk anyway and it's unhelpful or 'too much', I'll indicate) 'Not now' (verbally or by shaking my head
I've also moved position in the past - to sit on the floor.

It happens more with pdoc than with T I think, but that has more to do with session content than the persons involved.
(Now at least. In the beginning every pdoc session was a panic attack and I mostly said 'yes sir'. Well, I didn't say sir, no one says 'yes sir' around here as it sounds ridiculous, but you get what I mean. Pdoc is a doctor, I fear doctors, so I used to fear him)

Eta: or I just take a deep breath. Usually eyes closed. Breath in deeply, hold for about 2 heartbeats, exhale slowly. Then go on. If it isn't enough, I do it once more.
Also, I repeat what they say - slowly and in a few words - or ask them to repeat themselves/explain again. It's often enough to manage an immediate issue (sometimes issues, e.g. med stuff or agitation, have to be handled/discussed THAT session) and it requires less focus.

Also, bad jokes help. I make a bad joke or something and my pdoc or T will respond also with a joke, but within the sphere/scope of my joke - which I like, because it means they will joke, yet not make a joke that's inappropriate, triggering, unhelpful.