Pessimism that leads to a self fulfilling prophecy is irrational, but you still need a win. A win is fuel.
Choosing a belief that is less desirable than the most likely, is equally irrational, clearly pessimistic, and often self-fulfilling.
So the ideal belief system is irrational (optimistic) but only to a chosen and realistic extent.
Somewhere between Pollyanna and Eeyore, but more P than E. And as irrational psychologies go, moderate-P is by far the more successful of the two.