True and lasting joy comes from freedom, interior freedom. Christ has come to set us free so we have claim to this freedom. This doesn't mean we will never suffer or grieve, but that He has redeemed even our sadness. He became small and weak, an infant, so that we would dare to approach Him, unashamed of our own weakness.
So if you do not feel like rejoicing, offer Him your heart now (in all truth), right where you are at. He knows your weakness, he has experienced your poverty and wishes now to fill you.
"Everyone offering himself to God must offer the glory of life in himself, whether it be through giving it up or rejoicing in it; through a renunciation or an embrace...Everything can be put into the fire that Christ came to kindle; and whether it be the bitter wood of sorrow or the substance of joy, it will burn upwards with the same splendour of light." (Reed of God by Caryll Houselander, p.66)
Offer Him all and He will give you joy (perhaps alongside your sorrow), the joy of being His, the joy of freedom.
- Sister M. Annuntiata, FSGM