Why does a girl feel distressed, pessimistic about the future, and socially isolated after committing to abandoning masturbation and regularly performing prayers? Does this mean that Allah has not accepted her repentance?

Masturbation is forbidden and a grave sin, as is neglecting prayer, which some scholars consider to be an act of disbelief for those who abandon it out of laziness. You have done well by repenting from masturbation, maintaining your prayers, and steadfastly obeying Allah. If you are sincere with Allah and patient, you will find a good outcome, as Allah Almighty says: "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life, and We will surely give them their reward [in the Hereafter] according to the best of what they used to do." The feeling of distress may be from Satan or a trial from Allah, so do not pay attention to it. Seek refuge with Allah and be diligent in remembering and thanking Him. What happened does not necessarily indicate that your repentance has not been accepted. You must have a good opinion of Allah, for He accepts repentance, and His mercy is vast. Therefore, cast away these worries, accompany righteous women, attend gatherings of goodness, and frequently make supplications such as: "O Allah, I hope for Your mercy, so do not leave me to myself even for the blink of an eye, and correct for me all my affairs. There is no god but You."