All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If your maternal grandfather is the one who nurtured you, then you should treat him with kindness in return because being faithful and returning one’s goodness is among the traits of the generous and kind people; Allaah Says (what means): {Is the reward for good [anything] but good?}[Quran 55:60]
However, the fact that he was kind to you does not mean that he is permitted to prevent you from your right to get married. Rather, none of your guardians is permitted to do so to you, let alone your maternal grandfather who is not a guardian in principle, because the guardians of a woman are her relatives from the father’s side.
On the other hand, if a woman was mistreated by her ex-husband, this does not necessarily mean that she will be mistreated by her new husband. Therefore, if this man is pleasing in his religion and moral conduct, then your grandfather should accept him as a husband for you. Abu Hurayrah narrated that the Prophet said: "If a man whose religion and moral conduct are pleasing to you proposes to you, let him marry; otherwise there will be great mischief and corruption on earth." [At-Tirmithi]
Hence, you should be wise in endeavoring to convince your grandfather to agree that you marry that man, then if he is convinced, all perfect praise be to Allaah, and if he is not convinced, then you should marry him [that man] even though your grandfather disagrees, and he is obliged to expiate for his oath.
However, you are not permitted to marry except with the permission of your guardian who is your father. If there is an impediment for the latter to be your guardian, like being mad and the like, then your guardianship moves to the person who comes next after him who is your paternal grandfather, and then to other consecutive relatives that are mentioned by the jurists in Fatwa 83338.
According to the view of the majority of the scholars it is not a condition that the guardian be trustworthy (of good reputation). Moreover, according to the view of the majority of the scholars it is not permissible for you to marry without the consent of your guardian, contrary to the view of the Hanafi school.
Finally, it should be mentioned that you are still obliged to be kind and dutiful to your father regardless of what happened from him, and you are obliged to keep ties with your kinship from your father’s side in the same manner that you keep ties with your kinship from your mother’s side.