Jesus transformed the sadness of the betrayal announcement to the joy of singing by talking about the Messianic Banquet and his commitment to bringing it about. Future hope is essential for having enough joy. And amazingly, God does not hold our future sins against us now.
Jesus predicted the disciples’ fall citing Zechariah, which says that fall must happen for purification. Let your failures purify you from pride. They didn’t fall because they lost their shepherd. They didn’t lose him; they left him. He led them somewhere they weren’t willing follow (to death). When you fall, recover by letting it teach you humility and avoid falling by deriving joy from future hope.