A father is a guide and a protector for his child. He is a real life hero who can fight with the world to make his child happy. But if he gets disappointment from him then it is something which no one in the world except his child can rectify. So if you ever did anything which made him sad, then you should apologize for it then and there. It is better to express your apology in written. You don’t need to be formal, just write what you want and what you feel.
Here is a sample sorry letter to father:
I am writing this letter to you because I don’t have the courage to face you. Through this letter I want to apologize for disappointing you. I know you have many dreams for me and you want me to work hard to fulfill them. You want me to be a successful engineer in life and want me to be on the top of the world. But my last semester result was a big jerk in the road of my success and that had troubled you a lot. You were expecting me to score the top position but I am sorry for being not up to the mark.
I still remember all those days when I was preparing for the IIT entrance exam. It was the time of high stress and I was almost mad because of the stress. It was only you and your support that helped me in getting out of it and revitalizes me to fight the competition with great zeal and belief of achieving success. When I made it you were on cloud nine. I know all my clarifications are my excuses so I will not give any but I can only promise you that I will turn every stone to make your dream come true.
Dad, please forgive me because you are my strength and if you are not with me I can’t even move a single step.