Is a wife entitled to refuse marriage after her husband's death for fear that she will not be his wife in Paradise, and for fear that she will not fulfill her duties towards a new husband?

You are experiencing circumstances similar to those of Umm Salamah, may Allah be pleased with her, who suffered from separation from her family and the loss of her husband, Abu Salamah. Despite the severity of her affliction, she recited the Prophetic supplication: "O Allah, reward me in my calamity and replace it with something better." So, Allah replaced it for her with the Messenger of Allah, peace and blessings be upon him. Therefore, we advise you, O female servant of Allah, to recite this supplication frequently. And when a suitable and honorable husband comes to you, do not hesitate to accept him, for he will be a helper for you in obedience to Allah, in chastity, and in raising children, especially amidst tribulations.