Is your Mum able/willing to stop perseverating on something when your physical condition intrudes? Like if you had to run to the toilet or if you couldn't breathe, is she responsive to you saying 'I can't cope now, can we take it up tomorrow?" If yes, then the trick is to get her to see 'too tired to think' as a physical urgent crisis.

My former partner (yeah that one) was difficult to convince that we really would be willing to resume the hard convo later. With him we developed the convention of offering the other person the last word. "I have to go to bed, do you want the last word?" Also, we had to be rigidly rude about not talking at all after we had asked him to stop.

With your other friends, what would you do if they ignored your cues that you'd had enough? I think it's hard to be the same kind of assertive with one's parent, but at the same time, hard to expect them to pick up your cues.

*sympathy*