Is an abundance of supplication and a feeling of comfort an indication of the imminence of relief and the acceptance of prayers, or is it a whisper from Satan? What is the indication of a change in state after recalling past joy during times of distress? And are there specific signs that indicate the imminent acceptance of prayers?

The comfort you feel might be a good omen due to abandoning sins and drawing closer to Allah. Indeed, comfort and happiness are only achieved by drawing near to Allah, as stated in His Almighty قول: "Whoever does a good deed, whether male or female, while being a believer, We will surely grant them a good life." There are no specific signs for the imminent answering of supplications. However, it is advised to have good expectations of Allah, to desire His grace and seek His mercy, and to entrust all matters to Him, for He knows what is best for you, and to supplicate frequently.