How can one overcome the feeling of guilt towards their mother after secretly marrying her sister's son (maternal cousin) due to the mother's rejection of their marriage because of family disputes, especially given the difficulty in conceiving children, and should the mother be informed of this marriage?

Marriage of a woman without a wali (guardian) is invalid according to the majority of scholars, and the view of the majority is the most authoritative. The mother does not have guardianship over her daughter's marriage; rather, guardianship in marriage is exclusive to males. The most rightful people to give a woman in marriage are her father, then her paternal grandfather, then her son, then her full brother, then her half-brother (from the father's side), then their sons, then her paternal uncles. Therefore, if the man married you without a wali and a legitimate judge did not rule this marriage to be valid, then the contract is void. The correct way for you to be together is to renew the contract in the presence of your wali or his representative. The wali does not have the right to prevent a woman from marrying a suitable partner; otherwise, he would be unjustly preventing her marriage. In such a case, she has the right to refer her matter to a judge to marry her off or order her wali to do so. You should strive to convince your mother to agree to your marriage to this young man and seek help from someone who has influence over her. If she insists on refusing, you are not obliged to obey her in this matter, but you must honor your mother, treat her well, and obey her in what is good.