How can one get rid of extreme nervousness and mood swings that affect the person and those around them, especially when there has been a prior intention to get rid of them, but without success?

It is Allah Almighty's way in this universe to link effects with their causes. The nervousness you feel must have a cause. Therefore, it is necessary to fill your free time with what benefits you in matters of religion and worldly life. All goodness lies in faith, obedience, and good character, for through them, the happiness of this world and the hereafter is achieved. It is characteristic of a believer to trust in Allah, rely on Him, and entrust all matters to Him. One should be keen on accompanying good people and attending gatherings of knowledge, and beware of associating with transgressors and attending bad gatherings.