What are the pieces of advice for strengthening faith, focusing on memorizing the Quran, and avoiding forgetting it, especially when feeling distressed upon being preoccupied away from it?

Among the things that strengthen and affirm faith are: supplication (dua), abundant acts of worship (such as fasting, charity, and maintaining kinship ties), listening to the lessons of scholars, and good companionship that assists in goodness and remembrance. It also involves patiently keeping oneself with the people of piety and avoiding the people of misguidance, in accordance with His, the Almighty's, saying: "And restrain yourself with those who invoke their Lord morning and evening, seeking His countenance. And let not your eyes pass beyond them, desiring the adornments of the worldly life. And do not obey one whose heart We have made heedless of Our remembrance and who has followed his desire, and whose affair is ever [in] neglect." (Al-Kahf:28).

As for forgetting what you have memorized from the Qur'an, its remedy lies in frequent recitation and regular revision with a daily portion (wird), praying with it in supererogatory prayers, and finding good companionship with whom you can revise the Qur'an. Your feeling of distress when you are preoccupied from the Qur'an is a blessing from Allah.