Mental faint
Another way of expressing "mental breakdown" whether you use it as a noun or an adverb (mentally faint or faint mentally).
I am losing it and am going to have a mental faint.
A major mental faint is imminent.
I am going to mentally faint.
I am fainting mentally.
A major mental faint is imminent.
I am going to mentally faint.
I am fainting mentally.