Despair is a sin against the virtue of hope. Sloth is more an opposition to fortitude. I see the good, but it is difficult, so in my sloth I fail to act for the good I see. It could be something small like getting out of bed to go to mass, or it could be something great like becoming a missionary. Fortitude is necessary no matter how advanced in the spiritual life we are. It moves us to the next level, whereas sloth keeps us stagnant or can even move us backward.