Why do I feel drowsy when I remember Allah frequently, and what is the prayer for forgiveness that alleviates sins?

Remembrance (Dhikr) is one of the best ways to draw near to Allah. What happens during remembrance may be from Satan to distract you from worship. If you feel that, seek refuge in Allah and renew your ablution. Seeking forgiveness (Istighfar) in any form erases sins. The greatest form of it is "Sayyid al-Istighfar" (the Master of Seeking Forgiveness), and whoever recites it with certainty and dies on that day or night will be among the people of Paradise.