first up, why on earth would you say you have no right to feel robbed?? Apart from the fact that that's how you FEEL, and everyone has the right to feel however the hell they want, of *course* you feel robbed! Your family life was often a rough and rocky road, and one of the few reasonably constant things in that family was your dad. And he got taken from you when both you and he were so young - which, "mysterious ways" or "fate" or whatever be damned, IS a robbery of sorts. Not one you can *do* anything about to be certain, but just as I feel robbed of what should have been three to five more years of Sophie, you *should* feel robbed. I think it's a part of the grieving process - a sort of bitter rumination over the years that could have been. That's how people are - some portion of our relationships with the people we love is tied up in the fact that we expect them to BE there for some unspecified period of time, and when that gets yanked away, it's like not just the person (or dog) has died, but this whole set of (perfectly normal and respectable) expectations and dreams and images of the future have died too.
As for the rest of your post - I really do think you come down a bit hard on yourself (surprise!). Me personally, I kind of had to accept a long time ago - when I was still in therapy in CT, actually - that I'm just not ever going to be the kind of person that is happy or even content, not in any thorough sort of way. And that's part of who I am, even while it's made my life a zillion times more difficult.
BUT, one little thing - that voice? What it's telling you to do is what you choose to hear. Meaning, almost everybody has that voice to some extent or for some part of their lives, it's the "you're stupid, nobody loves you, look what you just did you idiot" voice. What you're hearing is the way you found to cope with things for a long time.
All I'm trying to say is that yes, that voice is probably not ever going to go away. I don't have a voice anymore but not because I'm better, just because my personal manifestations have changed greatly since I was a teenager. But you CAN translate that voice differently. Stick a different fish in your ear, so to speak. It probably won't ever be a happy shiny lovey pretty voice, but it can say other things, some of which can be helpful.
Personally I am coming to believe more and more these days that 99% of human behavior is rooted in biological imperatives. I think that voice is a combination of two things: first, the necessary apparatus of fear and morals that are genetically selected for in humans, gone awry in the cerebral cortex. We, like all animals, need to feel fear and regret out of a biological and evolutionary set of requirements (animals without fear don't live very long), and like other traits in us and other species, some of the means to those traits come along with some side effects. The second thing I think it is is a way of testing oneself. Pushing the mettle. Succumbing to that voice is a pretty quick means of natural selection.
It's easy to think that you're the exception with what you're talking about here - and often it is actively *comforting* to feel that way, to feel unique in one's suffering is something held dear to most people - but I guarantee you that the majority of people in this world, and not just the ones on meds or shrink's couches, have some manifestation of what you describe. Indeed it is the very rare and possibly entirely clinically insane exception that couldn't empathize with this to some degree.
And therapy and meds. You'll do what you need to do when you need to do it. If you FEEL like you ought to push yourself harder, and I mean feel that personally and internally, not because of some artificial set of external "what you oughta do" reasons, then you need to push yourself harder. But if you feel that you are moving at the pace you need to move at, then so be it, and screw anybody who says otherwise.