Is the distress, hardship, and difficulty the questioner is experiencing in her life, such as not getting married and having a bad relationship with her mother, due to a sin she committed, and how can she be sure that Allah has forgiven her? And how can she change her life for the better and feel the companionship of Allah?

What happened to you is a reprehensible act and a means to actual zina (fornication/adultery), but it is not the zina that necessitates the prescribed punishment (hadd). Your remorse and repentance are evidence of Allah's acceptance of your repentance, for Allah forgives those who repent, believe, and do righteous deeds. Your distress and anxiety may not necessarily be due to the sin; it could be a trial that raises your ranks and expiates your misdeeds. Be patient and increase your remembrance of Allah and supplication, for patience and remembrance bring tranquility. Marriage is a provision from Allah, so supplicate to Allah to grant you a righteous husband, and seek him while adhering to Islamic guidelines. Slight disfigurement does not prevent marriage. Strive to mend your relationship with your mother and treat her kindly, and seek help from those close to you to offer her advice, and beware of being undutiful to her.