I don't think this means an inversion
What comes first? - belief or achievement or self-confidence?
You won't achieve something if you don't believe in it. You just won't persevere and will give up or find workarounds.
But if you believe in it and keep failing consistently, your self-confidence will go down and so will your trust in self. It gets to that miserable "I'm no good" belief until one breaks out of that.
Once you achieve something, it will boost your self-confidence and trust in self will rise and your belief is validated. Then your belief will make you try more and achieve more. Sometimes it leads to overconfidence and bouts of overconfidence will break that trust within oneself - and we just need avoid getting into that downward spiral.
Many times "starters" define limits that are just self-imposed limits. People fall into the ugly 'I can't do that' loop without trying. And they have to be forcibly, inconveniently and painfully pushed out of that. This quote from Star Trek sounds so true "stallion has to be broken to reach it's potential"
I'm a pessimist but I don't believe that pessimism motivated achievements can make one happy unless it is a side-effect. Such achievements root from a negative inner rebellion behaviour. And if we aren't truly happy with those achievements, that makes us doubt and kill our own inner beliefs that led to those achievements. I'm not saying we should be a blind-optimist but stay close to reality.