How to stand up to my mom?

Fanny
Okay I'm 17 years old soon to be 18 in 2 months. I have a bf that my mom does not like. She hates him for no reason. We almost broke up because of her and because of distance. He told me that I need to stand up to her because I need to grow up and it's true. I'm relying on her so much and I think it's time to stand up to her but I'm still not sure. How do I do it without disrespecting her. Please help me out. Thanks.