It's definitely informal (but so is every other suggestion here so far)...
Tell it like it is - speak frankly
Tell the truth no matter who is criticized or how much it hurts.
Describe a situation honestly without avoiding any unpleasant details.
(I've highlighted the first definition as a less informal alternative.)
Note that tell it like it is fits all contexts where OP's not hold myself back might be used. It can, but doesn't necessarily, imply hostility/aggression, whereas give him a piece of my mind always does.