Whether we study Torah casually or on a steadfast and regular basis, we will find that no matter how much we toil and review our studies sooner or later we forget much of what we have learned. Many people get frustrated, in others the fervor of studying Torah is diminished, yet others get disheartened and abandon studying altogether.
The Sages of the Land of Israel read this property of Torah study right into the Scripture that commands us to study the Torah: "The Torah that Moses commanded us is the inheritance (which they read as "tiresome") of the Congregation of Israel" ( Deut. 33:4) (Bava Bathra 8:2)
The Sages of the Land of Israel teach us that the tiresomeness of Torah study is in fact constructive and is actually by design. Forgetting our studies should not be taken discouragingly but should be welcomed because it causes us to be perpetually involved in Torah study, keep it relevant to our heart and reap its rewards (Eccl. Rabba, 1:13).