Well, he was totally right, after years of trying, the business totally flopped and I had lost all of the money, as well as a ton of my own. It set my finances back at least a decade or more. Luckily, my dad has a lifelong friend who was willing to buy out the loan when it looked like the company was going to default, so now my debt was really to my dads friend.
Fast forward many years... my dad has retired and needs money. He's had trouble selling the property (although it looks like that is happening now in the next month or two). The property gained a lot of value over the years, so that is good. He will have enough to pay back the friend in full and a bunch left over for himself.
I've been paying my dad back as I can over the years and in recent times, reached the principle amount of the loan... now I'm paying the interest (the principle has ballooned more than 2x).
My current payments have been a lifesaver for him. It worked out that being able to pay him back now when he needs money the most, has been really helpful for him. Had I paid him back in full earlier, it might have been harder for him to come to me asking for a loan, now he has some steady cash flow from me. When the property sells, his friend will get paid back first and I'll just owe my dad the rest of the interest.
This has weighed on me for a long time and is something that I think about almost daily. It sucks owing money. I have no other debt than this. Some day, I'll be free of it and I can't wait for that day.