The theme of "letting go of old grievances" is best seen in Lords Capulet and Montague's behavior towards each other, especially with regards to the longstanding feud. The First Prologue makes it very clear that the feud is actually an ancient one, going back many generations, possibly even centuries. But the prologue also makes it clear the feud had been buried for a while but that now the present generation has resurfaced the argument, as we see in the line, "...From ancient grudge break to new mutiny" (First Prologue, 3). Lords Capulet and Montague would be considered the instigators of the new battles. Plus, since the feud has a long history going back many generations, the fact that Lords Capulet and Montague have started the fighting once again proves that they are unable to let go of old grievances, which helps portray the theme.
Even Prince Escalus accuses Lords Capulet and Montague of being unable to let go of old grievances when he breaks up the whole-city riot the Capulets and Montagues have created in the very first scene. Escalus states that three times now Lords Capulet and Montague have started "civil brawls" due to the exchange of trivial words and even incited Verona's elderly citizens to take out their old weapons that had been rusted from peace and take sides in the feud again, which has been stirred up by the two families' old and rusted hatred for each other. In other words, while peace had been prevalent, the Capulets and Montagues have started the battling all over again, and Verona's elderly citizens, who knew what the feud had been all about, are joining in by again taking sides on the issue, creating whole-city brawls, as we see in the lines:
...And made Verona's ancient citizens
Cast by their grave beseeming ornaments
To wield old partisans, in hands as old,
Canke'd with peace, to part your canker'd hate. (I.i.88-91)
Since the Capulets and Montagues have started the fighting all over again when there had been peace, influencing the elderly citizens to join in as well, their behavior perfectly portrays the theme of being unable to let go of old grievances.