Yes, I so much agree with the above post.
I know it is not easy, but you need to try to separate what is being said to you by your Dad, from the reality of the truth that so many here have pointed out.
Your Dad may be a nice guy, but he has some serious issues and is taking them out on you. In my opinion , you should indeed tell him how he makes you feel. But you also need to separate yourself from this when he does this. You cannot solve his issues, and you also don't need to be his punching bag. I hope you will protect yourself from this.

I know from experience how this parental/child relationship can be complicated. I hope that you will try not to do what I did: for over 30 years I let my mother's inadequacies make me feel bad. Finally I am coming to to terms that it was not my fault; she was a bad mother. If I could choose my family, I certainly would never have picked her for a mother. But you CAN choose your friends, and you can choose not to let your father make you feel bad for things which of course are not your fault.
There is no way your grandfather would want that for you; you never knew each other, but do this for him as well as yourself.