The state’s highest court has awarded a new trial to a Brockton woman convicted of fatally stabbing her boyfriend. Solange Anestal is serving a life sentence without parole following her 2007 conviction for killing Baby Petitry. The Supreme Judicial Court said Tuesday that the trial judge erred by letting prosecutors introduce evidence of the defendant’s prior bad acts. The court also said jurors should have had instructions about excessive use of force in self-defense. Anestal, now 33, argued that she was not criminally responsible because of mental problems following years of abuse.