Fortitude is a means by which unhappiness is overcome and satisfaction, happiness, nirvana and parinirvana attained.

How is fortitude a means by which unhappiness is overcome?

Through despair one feels and believes that one is unable to overcome unhappiness and attain satisfaction, happiness, nirvana and parinirvana.

And through despair one ceases efforts to overcome unhappiness.

Through fortitude one feels and believes that one has the power to overcome unhappiness and attain satisfaction, happiness, nirvana and parinirvana through perseverance.

An effort must be made and maintained to overcome unhappiness.

And it is through fortitude that one maintains the effort to overcome unhappiness and attain satisfaction, happiness, nirvana and parinirvana.