Diminish means become less severe
Well.... Tybalt is Juliet's cousin. Although he is mischievous and a bit of a rogue, he is still well-loved by Juliet and the rest of her family (the entire Caulet clan).
Romeo is now a murderer (forcing him into a rapid departure), and a murderer of a member of Juliet's family to boot! If things weren't already against him in blood alone, this act would certainly sour any advancements he could have possibly made in asking Juliet's father for her had in marriage.
She harms herself to prove to Brutus that she is strong enough to keep his secret.
She feels like she is not really Brutus's wife if he cannot be honest with her.
She thinks that Brutus is unfairly keeping her in the dark because she is a woman.