The biggest lies ...

Are the ones we tell ourselves.

And, with the occasional Hitlerian sub-humanizing lies, these are lies that primarily affect ourselves, and they're primarily internal.

No, they're not Dunning-Krueger effect lies about how wonderfully skilled and intelligent we are in cases where we're not Lake Wobegon above-average geniuses. (That said, they would come in second place.)

No, I see what I call motivational lies as the biggest lies. And, they are?

They're kind of the reverse of Aesop sour grapes lies.

We instead tell ourselves "I really want to do X" before venturing out to do X, or a larger task that in part includes doing X. Variants could be "I'll do X better this time" if we've had problems with X before and similar.

These are the type of internal lies that easily can be flipped to sour grapes lies at some point in the future, of course.

Example: "I really didn't want Y so badly as to work that hard on X," or "Y wasn't worth that much work on X," or similar.

But, we tell ourselves these original motivational lies when we're in a place of partial ambivalence. And, often, the ambivalence is more emotional than intellectual. We can't, or don't want to, commit fully to the work to do Y because something just doesn't feel right about Y. Per the likes of Antonio Damasio, we need to unpack those feelings first, before we go down the road of motivational lies to ourselves.