To understand why Jeremiah is so despondent in today's first reading, it's essential to appreciate the role of biblical prophets in the history of salvation.
Contrary to popular opinion, they normally don't predict the future, certainly not the coming of Jesus. Unfortunately this false notion of their ministry is often reinforced in our liturgies. The Second Advent Preface, in the old translation, stated, "(Jesus') future coming was proclaimed by all the prophets." In the revised translation, it still says, "For all the oracles of the prophets foretold him."