What is the reason for the decline in my ability to memorize the Holy Quran after I used to memorize it actively, and is that related to telling people about my memorization?

Memorizing the Holy Quran is among the best deeds, and sins deprive a servant of good and knowledge. Therefore, you must strive in repentance and pray to Allah to help you memorize the Quran. If you fear envy (hasad), you should thank Allah for His blessings and conceal them from the envious as much as possible, knowing that every doer has a period of activity, and the successful one is he who perseveres in good deeds, even if they are few.