Witnessing abuse as a child is very distressing and the effects of such so often carry over into adulthood, manifested in emotional difficulties. Nobody likes to see their parents abused, especially by someone else they hold so dearly. This can make the whole process even more stressful.
Your father was supposed to play the role of the protector of the family, yet witnessing abuse challenges this and will inevitably make it difficult to have any kind of respect for such a person, especially when your mother was being abused. This can have a lasting impact on your emotions and feelings towards the perpetrator, in this case, your father.
You say that your father has changed now, alhamdulilah, but you have a hard time forgiving him for the past. This makes it difficult for you to feel any respect for him, even if he has changed. Forgiveness for such terrible behaviour towards your mom and lasting effect it has had on you can seem like quite a difficult task, but there are some things you can think of that might make this a bit easier.
Think of any good times you had together and remember these times. Reflect on them and reflect on the good qualities he had, rather than focusing on all the negative memories. He is your father, regardless of his poor behaviour. Regardless of your age, it is important to hold respect for your parents, unless he is doing something or encouraging you to do something that is against Islam.
You say he has changed now, so, in sha’ Allah, he no longer behaves in a way that is against Islam. Therefore, it is important to overcome negative feelings for him and pray that he remains a changed man, staying away from the terrible behaviour that he engaged in previously. Perhaps, the reasons for his change is that he feels terrible for what he has done in the past, and is begging for Allah’s (swt) forgiveness, in which case he will need the support to remain straight and avoid the same behaviour again. Even if you don’t feel any respect towards him now, you can try to force yourself to behave in a respectful way. Eventually, it will come natural as the respect for him grows as you build relations with him again.
Remember that we all sin; some more than others, and some bigger sins than others. People hope for the forgiveness of others, especially the forgiveness of Allah (swt). When someone has wronged us in particularly, we need to remember that how can we expect Allah (swt) to forgive our own sins if we are not willing to forgive the sins of others. Forgiveness can relieve a lot of pressure and stress that is carried around as a result of ill-feelings to someone who has wronged us. It can feel very liberating just to let go of it. Rather than continually dwelling on it, leave it to Allah (swt) and move on rather than being held back by such heavy feelings. Forgiveness can be a way to soften the heart. It does take time to forgive someone, especially for something so terrible, but keeping Allah (swt) in mind with a decision to forgive and move forward can make this a whole lot easier.
May Allah (swt) bring peace and contentment in the lives of you and your parents.
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