My father caused a great deal of pain. I am not sure he was a psychopath, but he is in the ballpark. He is a very successful business guy - boundaries which I would typically observe don't faze him a bit.
My brother and I were serious high school runners, 4:07 and 4:09 milers. We both went to school on athletic scholarship, with only modest success in college. We were entirely on our own at age 18 -financially, emotionally - everything, and did not eat well, live well, or balance academics well (we were very good students), but made it through, focusing on somehow too taking care of our mother, a really abused and fragile person.
My father, a man once well known in track circles before he went into business, would beat us bloody if we did not run well. I think his motivation went beyond living vicariously through his sons (although a part) - he really didn't want to pay college tuition or expenses - he didn't see the value in paying tuition and doing so would have infringed on his philandering, always a priority with him. He was and is very well off, but cannot control his spending, and is really self-centered. Rolex watches for him, for his girlfriends, the Mercedes, the expensive country clubs, they are objects of desperate, maniacal desire, to the point where he is often angry and irrational in his pursuit of such things. I likely make as much money now as he ever did, but poured my dough into Ivy League educations for my daughters and in spoiling them. It simply made me happy. (By the way, my father was an excellent athlete in his college days, but got thrown out of two schools for criminal behavior, finally finishing somehow with a PE degree at a mediocre state school).
The violence was significant - I am certain I was the only 9:24 9th grader two miler running with a big gash in the back of my head after it was beaten against the basement wall. I thought my times were pretty good until I learned of Eric Hulst's exploits. When Hulst died way too young, the descriptions of him as a kind, gentle and courageous soul really hit me hard - ok - maybe the 9:04 freshman two miler was what I somewhat wanted to be - but I really wanted to be the kind and gentle and humble soul. It has been a lifetime of work, and I have a long way to go still.
My father abused and beat my mother too - who reacted throughout my early teen and teen years by being in a constant stupor through severe drug and alcohol addiction. One of my teammate's parents essentially fed me throughout my high school years. My mother never left for our safety, although she should have. I reluctantly have let her off the hook over the years because she was so frightened by the abuse. My brother has not let her off the hook - he rightly sees it as child and teen abuse but the situation was just too daunting for her. She has cried herself to sleep at night for years over her parenting and drug and alcohol problems - I don't want her to feel pain, and no matter what, my mother is worthy of love.
My father hated me far more than my brother because I would not submit easily, and I spent my youth being called dumb, fat and lazy. Oddly, it made it easier for me to disengage. I never really communicated with my father past age 16, and well, I did not see poverty as poverty but freedom from abuse and opportunity. Accordingly, I don't hate him at this point, I just stay away. He actually likes it that way, as he thinks that given my academic achievements at the nation's best schools, I am a weak kneed elitist snob (my guess is that I intimidate him, really).
My brother was favored and had more trouble breaking away. He and I both became very successful, doing better academically and professionally (both grad and undergrad) than with running. And yes, we paid for it all - a blessing in disguise because it made us both feel really confident. My brother does spend every day hating my father - no doubt an immense motivating factor for him - he is a world renowned Phd economist and investor - but it makes me worry for his health, which is somewhat precarious.
I am no expert on therapy or counseling. My brother has gone through a lot more than I have, even though he was less abused (although still subject to a horrific experience). Abuse damages you. So if you start off behind, whether in terms of development, confidence, sense of self, creation of a moral center, or whatever, whatever assistance gets you to a state where you can obtain the humility to say, look, i am behind through no fault of my own, but I will work on catching up and develop a sense of self which happy people obtain, well, that is a start. During my worst moments I was in a small group counseling setting. One of the women in the group was a young widow who lost her husband in a fighter pilot accident. She was inconsolable. Over time, she repeatedly was simply nice to me - no sexual overtones - just nice - a pat on the back type of thing, a smile, which from her in her grief, made my heart sing. I never knew more than her first name, by design. Her kindness really turned the corner for me - simple kindness made me have enough humility to start really working at being happy. I just needed to be around kind, nice people. So whether serendipitous or not, I think counseling with an open mind can help. You still have to own it - counselors or therapists are only advisers.
I will say to the original poster there is something more pernicious about having a sibling be the problem. Once past infancy and toddler stage, we generally are conditioned to take care of our siblings. Intellectually we can distance ourselves from toxic siblings, but getting rid of the emotional burdens in terms of feeling somehow responsible to fix their awful behavior I think is more difficult. I say this in kindness and hope - I hope you too (and others in a similar situation) find happiness - and remember, runners are more disciplined than most - you likely can find a way.