Man, who isn't getting abandoned in this book? The people abandon God, God abandons his people, Jeremiah accuses God of abandoning him. This book is really heavy on abandonment issues.
But at the same time, it promises people that they'll eventually be taken back to God. Abandonment's just a phase where they seem to be stuck without God's presence, but he's going to eventually lead the exiles out of Babylon. God hopes that the experience of being deserted will teach the people to stop abandoning God. It's a lonely world out there.
Betrayal is almost as pervasive a theme as abandonment. But whereas abandonment goes both ways—people abandon God, provoking God to abandon his people—betrayal is much more of a one-way street. The people betray God by worshipping foreign gods, sacrificing their kids to Moloch, classic stuff like that.
The only exception to this general rule might be when Jeremiah rails against God, accusing him of betraying/abandoning him. Otherwise, God tends to do exactly what he says he's going to do—reward obedience, and punish disobedience.
Betrayal by worshipping other gods is definitely the worst thing you can do. It's a violation of Commandment Numero Uno, but the Israelites have a bad habit of turning to other gods, starting with that Golden Calf back at Mt. Sinai. Hard to believe how you could worship other gods when the Real Deal is living in the Temple down the street, right?
There's a lot of suspicion of betrayal in the book. Everyone was pretty anxious and fearful, what with all the predictions of imminent invasion and destruction, so everyone suspected everyone else.
Since Jeremiah spends so much time warning about God's wrath, the parts that focus on compassion and forgiveness come as a surprise and a relief. Although God acts like he's condemning Judah permanently, he makes it clear that that really isn't the case. He'll forgive them once they've served their time, and he'll lead them back to Judah, where everyone will obey God and live happily ever after (or at least until Roman times). Even the other countries that God threatens with destruction will be restored after their time of punishment has elapsed. So in the grand scheme of things, we're looking at just one phase here. It gets better, but it's probably hard for the people to see that in the face of all the calamity.
The people who demonstrate courage the most often in The Book of Jeremiah are Jeremiah himself and the people who help him—particularly, Baruch and Ebed-melech. Ebed-melech speaks up in support of Jeremiah while he's in prison and successfully wins his freedom, and Baruch risks his own life helping Jeremiah. Given how unpredictable the rulers are, it was a big risk to stand up to them on someone's behalf. A person could get killed.
Jeremiah personally demonstrates pretty amazing courage throughout his prophetic life. He delivers an unpopular message that he doesn't particularly want to give, and risks everything in the process. He's constantly getting arrested, thrown into pits, and threatened with death. His commitment to the cause has nothing to do with his own personal well-being. It's completely related to the need to tell the truth, since the word of God burns in him "like fire." He has second thoughts a bunch of times, but God encourages him to keep on keeping on.
This is what the book's about. God judges the people as disobedient and wicked, and hands out just punishments. As bad as Jeremiah feels about God's inflicting these terrible punishments, he knows the people deserve it. After all, you can't worship Baal and Chemosh and sacrifice children and expect God to just sit by and watch. Even though the Hebrew Bible is about the relationship between God and the… well, Hebrews, we learn in Jeremiah and other prophetic books that God judges all nations. Nobody gets out without serious damage being done, or at least predicted, when they mess with God's favorite nation. It's interesting that throughout the Hebrew Scriptures, other civilizations acknowledge the Israelite's God also; they just don't happen to worship him. He's pretty strict and hard to please, after all.
Everyone ends up suffering in Jeremiah. After the Babylonians invade, all of Judah is in convulsions. People either die horrible deaths or get sent into exile. The city of Jerusalem is rubble. The Temple's gone. The lucky people died in the attack; the others are left to starve. Later, the invaders themselves will also suffer when God turns the tables on them. Even God seems hurt and suffering thanks to his beloved children who turn their backs on him at every opportunity.
Jeremiah's suffering is laid out in detail. And he doesn't exactly suffer quietly—he cries out to God, lamenting his pain, and also cursing ever having been born. He's the "weeping prophet," a man whose message makes him hurt about as badly as his persecutors do. BTW, for the up-close-and-personal, blow-by-blow description of the suffering in Judah, check out the Book of Lamentations. NC-17 stuff.