What are the matters or pieces of advice that can be followed for supplication to be answered, given adherence to acts of worship and doing good deeds, and the presence of a problem of being unable to forgive certain people?

A believer should make obedience to Allah Almighty his utmost concern and goal, and whether his supplication is answered or not, he may ask Allah to make his supplication answered, by taking the means to achieve this, such as: adhering to obedience to Allah Almighty and avoiding His prohibitions, eating lawful food, and seeking out times of responsiveness. If he does so and supplicates to Allah, he will never be disappointed, for either his supplication will be hastened, or it will be stored for him in the Hereafter, or an equivalent evil will be averted from him, as long as he does not supplicate for a sin or the severance of kinship ties. Allah may favor some of His servants by making their supplication answered, and this does not mean that others are not accepted by Allah. As for the hatred of close people, the advice is to forgive them and pardon them, even if they have wronged you, for forgiveness benefits you more than it benefits them.