Sorry you're dealing with this too ...
Pete Walker says they're our toxic inner critic!
http://pete-walker.com/shrinkingInnerCritic.htm
I challenge them and what they are saying each and every time they rear their ugly heads!
Sometimes I will even verbally tell them "Shut Up" ... "That's Not True" ... "Go Away, You're Not Welcome Here Anymore", etc.
I then remind myself of all my good qualities and characteristics.
Hopefully one day, they will be gone for good and replaced with loving messages instead!