That’s all. Choose: weakness, or strength. You can’t succumb to weakness and hope to augment it with strength; just as choosing strength, but occasionally allowing weakness to overcome you with only lessen your ability to achieve your goals.
(And I’m not talking here about the weakness brought on by physical or mental struggles with illness, which is a different thing entirely. I’m talking about focused attitude, which, barring those illnesses, is within almost everyone’s ability to choose.)
It’s as simple as William James said: Pessimism leads to weakness. Optimism leads to power. And power is more than merely strength, even though that is one of the more obvious manifestations of it. Power is the ability to make change happen; it is strength plus influence, plus will. And no pessimist, ever, has any power. Pessimism is about giving away your power; optimism is about claiming and using it.
So it’s really that simple: be pessimistic and weak, or choose optimism, and claim your power. It’s always entirely up to you.