Thank you so much for writing. I applaud you for seeking help with this instead of just giving up on your relationship with your mother.
With only a letter to go on, I can offer only limited ideas. The most important one being this: People can’t fight if the other person doesn’t cooperate. You can’t change your mother’s behavior, but you can change your own. If you stop participating in the fights and instead ask her with genuine compassion what you can do to help, she will have to respond to you differently.
I’m also guessing that you caused each other a lot of hurt during your adolescence. An important step in healing is to acknowledge your part in it (without blaming her) and to share with her that you want to move into a more adult relationship with her.
This will take time and forgiveness and compassion for yourself and for her. If you can’t make progress, then I suggest that the two of you go for a few sessions with a family therapist for some help. The therapist can teach the two of you new ways to interact with each other that are healthier so you can start to enjoy each other.
I hope you will follow through. You are only 20. With some work on the relationship, you and your mother could have decades of a mutually supportive, loving relationship.
I wish you well.