Oh ditch the guilt hilda - we reap what we sow. If your Dad has been so difficult over the years then you are going to have mixed feelings about him - it is inevitable and unavoidable. It is not something for you to feel guilty about. You did not make him the man he is.
My Mum was a difficult lady and I spent years feeling guilty that I did not seek out her company and try to have a close relationship with her. That was until a friend said to me in a puzzled voice.."Why do you feel guilty? - you were just a child when you were first trying to deal with her and none of it was your fault."
I think it is a bit like parenting - I used to say to Mums who were finding it very hard that they should work out how much parenting they could do well and then seek out playgroups or childminders to do the other bits. It is the same with elderly parents - obviously we want to do the best we can, but we need to work out what we can manage with our own heath needs also in mind and then do that and pass the rest on to the care services or other relatives.
It does sound harsh, but mercy has to be tempered with self-preservation.
Even if we understand the forces that have made some parents difficult, it does not mean that the effects of their behaviour on us are any different. I finally, as an adult, decided that my Mum had severe PMT as she really was two people - very hard for a child to cope with.
I was very interested in a "Who do you think you are?" episode where Patrick Stewart (he of Star Trek fame) found out that his father had suffered from shell shock and this explained his violent and controlling behaviour towards his family - but Patrick also, in a very well-balanced and guilt-free fashion, recognised that having this explanation in no way minimised or mitigated what they all went through.