I'm sorry your mom isn't here to help you when you need it.
It's common that people who don't have good supportive parents find themselves with troubled partners. It's the feeling that we want to be needed and essential to someone. I've been there!
You don't need me to tell you that this is no way to live.
21 is young enough that you really need help and can't be expected to go through this alone. Talk to a counsellor and DV resources in the community, get some help so you can focus on yourself and your education.