I'm about to lose my marbles
Being in the the state of confusion and absolutely going bonkers. The person who says this phrase usually states it in a tone where they are rather stern and will get ready to square up on the spot (known from experience).
Shannon- Sweetie, are you alright? You're strangling the dog again!
Bertha- I'm about to lose my marbles!
Bertha- I'm about to lose my marbles!