Saint Isaac of Nineveh gives a quick key to changing sadness into joy. It’s really quite logical:
Hardships for the sake of the good are loved as the good itself.
So if all of our intentions are for God, then all of our hardships, pain, sadness, and suffering are equally for God, loved for God, and in love, transformed. Getting there and letting God have it all is not easy for us. But yet the central mystery is so simple.