Tips on having opening up to my mother.

As this semester winds down, I am now having to tell my mom the things i’ve been dreading. My boyfriend is living with my father (my parents have been divorced for 17 years) and I want to tell her because she has the right to know. She is a very controlling woman and she is very big on me and my boyfriend sleeping together at all. I am 20 about to turn 20 and he is 20 also. I want to spend lots of time with him this summer but i’m afraid she going to go crazy when I tell her he is living with my father. Any tips on how to bring it up to her and talking to her about it without getting kicked out?