No. If you used an AED promptly and correctly, you did everything right. Unfortunately, not every person in cardiac arrest can be saved, even with fast and accurate treatment. Some underlying medical or cardiac conditions mean that ventricular fibrillation cannot always be corrected by defibrillation, regardless of how quickly it is delivered.
Acting quickly and using the AED gave that person the best possible chance. That matters, even when the outcome is not what you hoped for. If you are struggling with the experience, please talk to your GP or contact SCA UK for peer support.