It is not permissible for a Muslim to accuse his brother in Islam of disbelief, if he is not so. He should repent to Allah and seek his forgiveness from this sin. Moreover, he has to ask the accused person to pardon him. The Prophet (peace be upon him) forbade this in many authentic Hadiths.As to the remedy of forgetfulness and the slowness of memorization, you have to fear Allah (Glorified and Exalted be He), continue studying and repeating what you memorize and ask Allah to support you with that. (Part No. 2; Page No. 139) We ask Allah to grant us all success in achieving what we want.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.