You are not doing anything wrong and should not be feeling guilty. You deserve a life of your own - you deserve happiness. Your are definitely old enough to be making your own decisions. Your mother at the end of the day is being selfish and clingy and she's trying to make you feel guilty by accusing you of being immoral and hating the family. It also maybe that she just doesn't understand you. She's also possibly afraid of letting you go too, but thats no excuse.
I guess the best you can do is have a serious chat with her as victoriangirl suggested and explain the reasoning behind your actions. Maybe explain how unhappy you have been before you met this guy and how much happier you are now and how it has nothing to do with any hatred or anything towards your family.
I hope things works out for you, but don't get your hopes up. Some people are just impossible. I don't personally think she will cut you off, but I can't read her mind. Is she upset with your boyfriend for any particular reason other than you spending less time with your family because of him? Is he from a different religion or something?
My situation is kinda the same (minus a girl and happiness, just clingy parents, yay
). So congrats on finding a partner that makes you happy
.