How do I stop forgetting what I memorized from the Quran?

Mostly by revising little and often. The Prophet ﷺ compared memorized Quran to a tied camel that runs away if you loosen the rope. Daily revision, reciting in your prayers and regular checks with a teacher or friend keep what you have learned fresh. Forgetting is human, so don't be discouraged.

Why memorized verses fade

The Prophet ﷺ said: "Keep refreshing your knowledge of the Qur'an, for I swear by Him in Whose Hand is the life of Muhammad that it is more liable to escape than camels which are hobbled" (Sahih Muslim 791). In another report he said that the one who keeps it tied will keep control of it, but if he lets it go, it will run away (Sahih al-Bukhari 5031). This isn't a threat; it is a reminder that memory needs maintenance.

Practical revision methods

Revise daily: set a fixed time, such as after Fajr, and review old material before adding new. Revision is more important than racing ahead.
Use your prayers: reciting what you know in your prayers, especially at night, keeps it alive. The Prophet ﷺ said that when someone who has memorized the Quran stands for prayer and recites it night and day, it stays in his mind (Sahih Muslim 789).
Create a cycle: divide your memorized portion into sections and review a different one each day, so everything comes round regularly.
Say it aloud without looking: reading from the page only feels like remembering. Test yourself, then check the page for mistakes.
Use the same Quran: the position of a verse on a page helps your memory, so stick to one edition.
Pair up: recite to a teacher, spouse or friend who can listen and correct you.
Listen: following one reciter again and again makes the flow of the verses familiar.

Understand what you recite

Verses are easier to keep when you know what they mean and how they connect to the one before. Read a short translation or tafsir of each section you have memorized. See How do I memorize short surahs for prayer?

Even the Prophet forgot

One night the Prophet ﷺ heard a man reciting and said, "May Allah bestow His Mercy on him, as he has reminded me of such-and-such Verses ... which I was caused to forget" (Sahih al-Bukhari 5038). This matches what Allah says to him:

سَنُقْرِئُكَ فَلَا تَنسَىٰٓ إِلَّا مَا شَآءَ ٱللَّهُ "We will have you recite [the Quran, O Prophet,] so you will not forget [any of it] unless Allah wills otherwise." Quran 87:6-7

So forgetting a verse does not mean you have failed. Go back to it, relearn it and keep going. Adults especially may need more repetition, and that is normal. See Can adults memorize the whole Quran?

Keep a sustainable routine

A short, steady routine beats a heavy one you abandon after a month. If you can only manage ten minutes, use them on revision. Match your daily reading to your real life. See How much Quran should I read every day?

Key points

The Prophet ﷺ said memorized Quran escapes faster than a hobbled camel unless you keep reviewing it.
Revise daily, recite in your prayers and test yourself without looking.
Use one edition, a teacher or partner, and one reciter to build familiarity.
Forgetting is human; even the Prophet ﷺ was caused to forget some verses.

Tip: Use a simple chart or app to tick off each day's revision. Seeing an unbroken line of days is a powerful motivator.