Why does a girl feel uneasy and reject all good suitors despite performing Istikhara and praying for a righteous husband, and why does it end in rejection from the other party in the few times she agrees, causing her and her parents sorrow and a feeling of tribulation?

There is no sin upon you for rejecting a suitor, even if he is religious and of good character, whether the rejection is due to lack of handsomeness or for other reasons. Do not fear punishment for this rejection. Have good thoughts of Allah and do not despair. If someone whose religion and character you approve of proposes to you, accept him and rely on Allah. Increase your remembrance of Allah (dhikr), perform lawful ruqyah, recite adhkar, and strive in supplication with persistence.