Well your mother sounds like an utter bitch in your adulthood as well as an active abuser in your childhood. What do you gain from having a relationship with her?
The situation is heartbreaking, you're right. She's nothing like what a mother should be and your father is nothing like what a father should be. They're both a waste of space IMO. it's a kind of grief, you're grieving the parents you never had, the realisation that you never will have those decent loving parents. It's a loss of sorts, a loss of hope I suppose. Even if they're still alive it's a loss you're feeling and it takes time to grieve any loss. It's a process and it'll get easier.
With ordinary grief at a loved one dying, you eventually reach a point you can look back at your memories and smile at the good ones, the happy times you shared and feel glad that you knew them. The grief of losing them is the price you pay for the love shared between you. If you hadn't known them you'd not have the grief but you'd also not have had the love. Recovering looks like moving on with your life without them in it, finding ways to fill the hole they left when they died. Hopefully with something equally as good, although obviously different, since they're irreplaceable.
With toxic parents the grief hits different. For a start a dead person can't help dying, they're not deliberately hurting you. Toxic parents who are still alive absolutely could choose to be different. It's a rejection. They'd rather remain toxic than have you in their life. They'd rather have been toxic whilst you grew up, abusing you rather than brought you up to be a well rounded individual and given you a solid foundation to start life from.
All grief feels like a raw wound, like you've lost a limb. Yet with ordinary grief from death you're aware you once had all the limbs. With toxic parents it's somehow even worse than that, because with the realisation of what they are (abusive), you've lost the opportunity for something fundamental that everyone should have: their parents love. The foundation you thought you stood on is an illusion. You never had their love, so you're not standing on a solid base with a raw wound that needs to heal, you're floundering in space with your feet not touching the floor. There's no solid foundation to pick up the pieces of a shattered life and try to get back to some semblance of normality. You never had normality and have to first figure out what it is before you can start trying to get there. There'll never be any happy memories to look back on. You may even have lost some, as it dawns on you that memories of them you previously thought of as happy times weren't really that, they were only the absence of (or perhaps just a reduction of) abuse at that particular moment. In this grief you're paying the price for a debt you never ran up, they did. There's no flipside to having known them because you never had their love. You were still a child who needed it though.
Healing from that grief doesn't feel like sadness fading and being replaced by warm feelings, it feels like indifference. As you start to heal and process all your emotions, the indifference grows until you no longer care about them at all, they become nothing to you, of less value than a stranger you've never met. Because that stranger has the potential to be a nice person and they don't. Recovering from this kind of grief looks like moving on with your life without them in it, by having chosen to cut out the poison that was them so you can become stronger and learning to build a nice life for yourself, to recognise toxicity in others and stay away from it in future.