DONE WITH CHEATING BOYFRIEND!!!!

So I caught my bf cheating, long story short I'm done with him! (I'm serious this time) the only problem I have is I'm VERY close with his family. I'm closer to his mom than I am with my real mom. She calls me her daughter, always buys me clothes, his grandmother calls me her granddaughter like im extremely close with them. In the past whenever me and him had issues, his family will sometimes mend things together or whenever I say I'm done with him I always change my mind once I get around his family because "sometimes" his mom will say things to me like "he really loves you" or she'll be on my side one minute and say "I think you should be done with him" which makes me feel a little better but whenever she switches up I always start changing my mind. Now that I want to be DONE with him for good how can I distance myself from the family? I feel like it will hurt me just as bad just because she is like a mom to me and I love her very much. But I think it's what best. I'm scared to tell her because I know she's gonna be very mad or say things like "just because you guys broke up doesn't mean anything. You'll always be my daughter" she even called me today so we can hangout and I didn't answer. What should I do? How should I go about this? Has anyone ever been in a situation like this? HELPPPP!!!!