Despite the love and respect Brutus had for Caesar, he decided to take part in the conspiration against him and assassinated the emperor himself. In Brutus's words in his speech in the Capitol, we can read "not that I loved Caesar less, but that I loved Rome more" as an explanation for his bloody deed. Since Caesar's ambition was dangerous for the Romans, Brutus slew him.
If sounds as if it refers to his aching agony within his situation. ¨Bear up, old heart! You´ve borne worse, far worse,¨ shows how everything is affecting him. This is his attempt to tell himself to stay strong in time of dispair.
