Most people see forgetting as a problem, but I see it as a great benefit.
If you never forgot anything, it would be impossible to serve God because remembering everything from the past, you would never be able to lift yourself up to God. Whatever you tried to do, you would constantly be disturbed by your memories of the past. But having the power to forget, you do not need to be disturbed by the past.
This is very important when trying to serve God. Most people are troubled by the past, especially when trying to pray. Precisely then their minds become filled with all kinds of distracting thoughts about their business, household and other affairs. They worry that maybe they did something wrong or should have acted differently. When a person tries to pray or study, he can easily become troubled by thoughts about past wrongdoings and personal failings. This is a universal problem, as we all know.
The best advice is simply to forget! As soon as something is over and done with, regret whatever you must and then push it out of your mind and pay it no further attention. Don’t even start rehashing things that are over and done with. Understand this well, because it is a very important principle.
In Torah study too, God gave us the power to forget so that we should always appreciate the Torah as we did the first time we learned it. Since you forget, relearning or reviewing a lesson is like learning it anew, and you appreciate it as much as the first time.
Imagine a group of men who have been hired to fill up barrels that are full of holes. Everything they pour into the barrels leaks out. The fools ask what purpose there is in trying so hard to fill the barrels if everything poured in leaks out.
But the wise man says, “What do I care? I get paid for each day of work. Even if the barrels leak, my wages won’t be reduced!” Similarly, even if you forget what you learn, your reward will not be reduced.
In the future God will make you remember everything you ever learned, even if you forgot it during your lifetime. The same applies to those teachings of the true Tzaddik that you do not understand. In the world to come you will be able to understand them.
In the life to come, your soul will remember and understand all the Torah you heard and learned in this world. Happy is the man who spends his days in Torah and devotion."