What should I do to repeatedly see the Prophet, peace and blessings be upon him, in a dream, after having seen him once four years ago and it has not recurred, especially with my feeling of despair and frustration?

The remedy for sorrow and grief is to seek refuge in Allah Almighty and to frequently recite the supplication for sorrow and grief: "O Allah, I am Your servant, the son of Your servant..." As for seeing the Prophet, peace and blessings be upon him, in a dream in his true form, it is a truth and an honor from Allah Almighty. There are no specific actions to perform to acquire this, but being guided by his guidance and following his Sunnah (definition) are among the best ways for a lover of the Prophet, peace and blessings be upon him, to be honored.