Put your feelers out for another job (easier while you are still working for someone else).

Diarise or make a log of all these jobs you couldn't complete, and the reasons why. Same for late/urgent changes to schedule at the boss's instruction.
Much easier to read or table these at the meeting so you don't get heated & say something you will regret.

Odds are this meeting will be your "formal warning", a precurser to when they decide to get rid of you. Do not argue, but also do not agree with any statements about your performance that you know to be untrue. As you say, take someone level headed with you.

I've been through this many times (from the other side of the desk).
It is good practice to have a witness for each party to prevent misunderstandings, vital to avoid unfair dismissal actions.

Good luck.

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A nation of sheep will beget a government of wolves.