I’m so sorry for your loss. There is no good time to lose a mother but some times are tougher than others. At just 20, you are just coming into your own adulthood. It’s a time when mothers and daughters often draw closer. You are aware of all that the two of you are going to miss sharing – your successes, perhaps a marriage and children. Of course you are mad and sad and don’t want to deal with it.
As you are finding, though, there is no way around the grieving process. You can’t somehow bypass it. You can’t avoid it or make it go away. The only way to resolve it is by embracing it. You need to do that for yourself. And – I suspect – it’s what your mother would want you to do. She wouldn’t want you to be stuck in this angry, unstable place. She would want you to remember the good times you had and the wisdom she did her best to share.
Please consider seeing a therapist a grief counselor or a clergyperson to help you. If you could have managed the grieving on your own you would have done so. There is no shame in having someone to guide you and support you along the way. Many hospitals and hospice organizations also sponsor grief support groups for people who share in this experience. People who are going through the same kind of thing will understand. Supporting others who are supporting you will also help your feel better.
I wish you well.