You seem to have quite a bit of compassion for your father, despite how miserable he acts toward you. That indicates emotional maturity on your part. So I'ld say you are already more than half-way toward having as good a relationship as it may be possible to have with this man. Maybe you could build on that by asking your dad to talk a bit about how he feels about some of the stresses you've described. It sounds like he could use a friend, and it's never occurred to him that you might potentially be the best friend he will ever have. I guess that amounts to some role reversal, but you may have to be "the adult in the room" when your father and you are alone. I'm sorry you have an emotionally needy parent to tend to right when you are at a stage of life when your father should be mentoring you. Sadly, he just doesn't have that to offer.

My father was emotionally needy and often distant, though not as extreme a case as your dad. I think there was more mutuality in my situation. It sounds kind of one-sided in your situation, which is unfortunate for you. But it can make you a stronger person to have to make your way forward without the support a well-adjusted father would provide.

Why you got to be the target of your father's frustrations, instead of your sister, is hard to say. But there are reasons. I don't believe human behavior is ever just random. In years to come you may gain more insight.

I'ld caution you against letting your father overly burden you emotionally. Needy people can do that. You could overly invest in trying to have a good relationship, just to end up drained and perpetually frustrated. I respect that you see reasons for your dad's unhappiness. But his life is the product of his decisions. He probably won't change his thinking and will probably always have a somewhat sad existence. That's on him. It's not your job to "fix" him. I would encourage you to invest in relationships with other people, who can reciprocate in a healthy fashion.

In short, you probably never will have "a good relationship" with your father. I'ld set the bar lower and shoot for one that is reasonably amicable. You can't keep worrying about it. It is what it is. The failure is his not yours. Don't hold yourself overly responsible.