I understand that you're very angry. However, a lot of these here aren't clear.

Is there a reason why your dad never found out that you were paying back this "loan" he gave you to your mom to give to him, while they were married and living together under the same roof? It seems strange that it was never brought up that you were paying him back and this was never revealed through coincidental communication all this time. Further, I don't mean to make you feel like your anger is not justified, however- your parents were a unit. It's not "your dad's money" and "your mom's money" because they were married. I'm not sure why you're separating the two. Your mom decided you owed them back the money your dad gifted to you. Honestly, it wasn't just his to gift to you because they were together. Yea it's nasty for her to single handedly decide that you owed it back without ever consulting him, but this isn't a cut and dry situation. Legally, she was entitled also to the money he gifted to you. It's not as simple as "this money was his to give." It wasn't. A jerk move, maybe, and selfish as hell, but this has no legal grounding at all to get back from her despite what everyone else is saying. Also, when he gave you this money, why was it not clear whether it was a gift or a loan? Sorry, but all this just doesn't make sense.

If family financial matters worked the way you are thinking, then my own mother owes me literal thousands. She had me paying cell phone bills, paying for clothes, food, and putting $1,000 each year beginning at age 15 towards a college education account that my father never wanted me to pay but she demanded it, and he never knew I paid for any of it. The money for it was already set aside. Eventually, she took money out of that college account without ever consulting me to pay for my brother's(more expensive) schooling. Money I had put into that account from working at g.d. Burger King. Surprise- now that I'm finished with school they have a bay condo! But it doesn't change the fact that if they were to divorce now, now matter how angering it is it would not make me entitled to back payments of all that money she took from me wrongfully. It just doesn't work like that. I think the moral of these stories is that if you are married to someone you don't let large sums of money going in and out of your accounts just go under the radar. It was your father's responsibility too to question her as to why she was charging you money and how that money magically showed up in their accounts.

Honestly you have every right to hate your mom for her selfishness, but you have no ground to stand on in getting the money back. The whole thing is a communication clusterfuck that you and your parents should've realized long ago. Get her out of your life and leave it at that.