One must always hope when one is desperate, and doubt when one hopes.

The counterweight goes in whichever direction you are leaning. When things are bad, the mind produces total accounts of ruin, and a little hope corrects a distortion rather than adding one. When things are going well, the same machinery projects the good stretch forever, and doubt is the correction there.

None of that is a recipe for permanent ambivalence. Ask which way you are currently tipped and apply the other. Feeling is not evidence, and the strongest feelings are the ones most in need of a second opinion.