You have every right to stand up for yourself and your happiness here. It sounds like going low-contact with your Dad is something you want and I think that would help. What this looks like depends on when/where you have contact now. Do you see your parents only on holidays? Have regular phone calls? Whatever it is, move towards your desired level of contact.

With your Mom , it's time to have a talk with her about how your contact with your Dad is going to change and why. She needs a heads up on that, as it affects her too. Just because she puts up with his behaviour doesn't mean you have to.

As far as using her as your emotional dumping ground? I mean, check yourself that you aren't spending all your time griping to her while not asking her life at all, but I very much doubt that is the case. Your Dad's bullying remarks are just that. Don't let him dictate what you tell your Mom about.

And finally - if you have access to any, therapy may help you process your emotions about your family situation. This has been going on for a long time and it might help to talk to a professional who can help you contextualize it and offer support. Good luck, OP.