This is an interesting question that can be answered in various ways, because the idea of "losing heart" can be interpreted in a few different ways.
First, if we define not "losing heart," as keeping faithful to a plan, then Napoleon and Squealer would be great candidates as examples of not "losing heart." They wanted the rebellion to take place, and they were aiming for power. They get this power, and they hold on. Moreover, by the end, they have absolute power in the most underhanded way. By chapter 9, Animal Farm is a Republic with Napoleon as president, even if he was the only candidate. In the end, they experience a transformation. They are humans. In view of this, they certainly have not lost heart.
Second, if we define not "losing heart" in terms of sincerity, Boxer is the best example. To his death, he was faithful to the rebellion. He literally worked himself to death.