What is the interpretation of the doubts that assail a girl who has repented, increased in acts of obedience and seeking forgiveness, and relied upon God concerning her marriage, despite her certainty in God's power? And how can she rid herself of these doubts to be more reliant and trusting in God?

To proceed: We ask Allah to accept your repentance, strengthen you, and grant you a righteous spouse. Psychological discomfort may be due to a deficiency in the remembrance of Allah and obedience to Him, for "unquestionably, by the remembrance of Allah hearts are assured," and "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life."

If you do not find the fruit of that, it may be due to a deficiency in the conditions for supplication, or the presence of impediments. For Allah answers whoever supplicates to Him, and what is required is to fulfill the conditions, have certainty of the answer, and not be hasty. Doubts may be satanic whisperings, so seek refuge with Allah from them, and ponder the texts of the Quran and Sunnah and the biography of the righteous predecessors to increase your certainty in Allah. Seek help from your righteous sisters in finding a husband.