But how the hell do you "believe in yourself"?

Morendil's given one meaning that's useful; another one is, "assume that you'll be able to handle the (likely) worst-case results of your actions, so that your decision making isn't paralyzed by implicit fears."

Btw, I used to think that doing these sorts of translations were all that was needed for self-help to be usable by geeks, but that's not actually the case: being able to understand a piece of advice (like this one or Morendil's variant) is not at all the same as being able to implement it.

In practical terms, the advice I've just given usually requires one to let go of many existing beliefs or fears, while the one Morendil gave is a skill that requires practice, and may also require letting go of the same beliefs or fears. In neither case is the mere understanding remotely sufficient to accomplish anything except a feeling of having insight. ;-)

(Btw, in general, when self-help advice says to "believe" in something, it actually means refraining from disbelief, i.e., you do not have to convince yourself of something that isn't true, but merely refrain from questioning it, just like one doesn't question the premise of a movie while enjoying it. Or, another way of looking at it, is that "belief" consists of thinking and acting "as if" that thing were true, i.e, "What would I anticipate and/or do, if I assumed that this were true?" Most other meanings of "believe" are irrelevant to implementing the advice.)