Learn English – Word for willingness to change your mind

Additive Inverse.

"This operation is also known as the opposite (number), sign change, and negation"

"Along its application to real numbers, "change of sign" is used throughout mathematics and physics to denote the additive inverse (multiplication to −1)" - Wikipedia (Sign)

The most common expression that describes the instance when you suddenly forget what you were about to say is

• your mind goes blank

I was just about to say something, but my mind's gone blank

Alternatively you could use

• lapse

I had an awful lapse of memory when I was asked to talk about X.

via @Noah

• slip your mind

I meant to tell her that Nick had phoned, but it completely slipped my mind