The problem of forgetting Quran
Question

When I was a youngster I was sent to a Boarding School and
did hifz for a while. I left the hifz classes after completing four paras (juz). This was due to the fact that I could not manage school & hifz at the same time. I was 12-13 years old at the time and not baaligh yet.
Is it a sin that I have now, almost 20 years later, forgotten the 4 paras (juz)? People have said it is a very great sin and that I should learn and memorize it again? 
I am confused. Please help!
Praise be to Allah.Undoubtedly forgetfulness is something that is natural in man, and man is only called insaan because of his forgetfulness (nasiy). Usually this varies from one person to another, according to the differences in the strength of the memory that Allaah has created in His slaves.
 The Quran “escapes” from the heart if the Muslim does not constantly and regularly review what he has memorized of it.
 The reason for this may be that it is a test of people’s hearts, to show the difference between the one whose heart is attached to the Quran and regularly recites it, and the one whose heart is attached to it only whilst memorizing it, then he loses interest and forgets it.
 The reason may also be to give the Muslim a stronger motive to recite the Quran more frequently so as to attain the immense reward for every letter that he recites. If it were the case that he could learn it and never forget it, he would not need to read it frequently and then he would miss out on the reward for reciting and reviewing it regularly. Fear of forgetting it will make you keen to recite it so as to gain more reward with your Lord. For every letter you recite you will have one hasanah, and the reward for one hasanah is ten like it.
 The Messenger (peace and blessings of Allaah be upon him) urged us to read Quran regularly lest we forget it. He warned us against being negligent in this regard, as was stated in numerous ahaadeeth, including the following:
 1-The hadeeth narrated by al-Bukhaari from Ibn ‘Umar (may Allaah be pleased with him), that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The likeness of the one who memorizes the Quran is that of the owner of a hobbled camel. If he tends to it regularly, he will keep it, but if he lets it go, he will lose it.” (al-Bukhaari, 5031).
 It is well known that if a camel escapes and runs away, it cannot be recaptured except with a lot of stress and difficulty. Similarly, if the person who memorizes Quran does not regularly review what he has memorized, he will lose it and will require a great deal of effort to get it back.
 * Al-Haafiz Ibn Hajar said in al-Fath (9/79), in his commentary on this hadeeth: so long as one constantly reviews it, what one has memorized will remain, as is the case with a camel, if it remains hobbled, you will keep it. The camel was singled out here because it is the most likely of domesticated animals to run away, and if it does run away, recapturing it is very difficult.
 2 – Muslim narrated in his Saheeh (no. 790 and 791) from Abu Moosa (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Read this Quran regularly for, by the One in Whose hand is the soul of Muhammad, it is more likely to escape than a hobbled camel.”
 3 – Al-Bukhaari (may Allaah have mercy on him) narrated that ‘Abd-Allaah said: the Prophet (peace and blessings of Allaah be upon him) said: “It is not right for any one of you to say, ‘I have forgotten such and such.’ On the contrary, he has been made to forget. Try to review the Quran, for it is more likely to escape from men’s hearts than camels.” (Saheeh al-Bukhaari, 5032).
 Al-Haafiz said in al-Fath (9/81): Ibn Battaal said, This hadeeth is in accordance with the two aayahs (interpretation of the meanings):
‘Verily, We shall send down to you a weighty Word’ [al-Muzzammil 73:5]
and
‘And We have indeed made the Quran easy to understand and remember’ [al-Qamar 54:17]
 So whoever strives to memorize it and recite it regularly, it will be made easy for him, and whoever turns away from it, will lose it.
 This is what encourages us to constantly review what we have memorized and to keep on reciting it, lest we forget it. The Prophet (peace and blessings of Allaah be upon him) gave this example because it is the best way to explain what he meant. He also confirmed it with his oath ‘by the One in Whose hand is the soul of Muhammad’, to affirm the importance of constantly reciting the Quran and reviewing what one has learned.
 3 – With regard to the report that Anas ibn Maalik (may Allaah be pleased with him) said: the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The sins of my ummah were shown to me and there is no sin greater than that of a man who was given or was helped to memorize a soorah or an aayah of the Quran and then he forgot it.” This is a weak (da’eef) hadeeth which was classed as such by al-Bukhaari and al-Tirmidhi. See Takhreej Mishkaat al-Masaabeeh by al-Albaani, no. 720.
 Imaam Ibn al-Munaadi (may Allaah have mercy on him) said in Mutashaabih al-Quran (p. 52):
 The Salaf were always afraid of forgetting Quran after they had memorized it, because this was a shortcoming.
 Al-Suyooti said in al-Itqaan (1/106):
 Forgetting it is a major sin, as was stated by al-Nawawi in al-Rawdah and others, because of the hadeeth “I was shown the sins of my ummah…”
 One of the best ways to help oneself to remember what one has memorized and to make it firmly-entrenched is to recite it in one’s salaah, especially in Qiyaam al-Layl. The Salaf used to recite it during the day and when praying Qiyaam al-Layl.
 If you strive hard to review the Quran regularly, there will be no sin on you even if you do forget some of it. The blame is on those who neglect it and fail to review it and read it regularly. We ask Allaah to forgive us.
 O Allaah, make the Quran the life of my heart, the light of my breast, a departure for my sorrow and a release for my anxiety. O Allaah, teach us from it that which we do not know and remind us from it of that which we have forgotten, for You are the All-Hearing, All-Knowing.