You are totally doing the right thing.
Kudos to you for sticking to your guns. I understand it must be difficult, it sounds like your daughter is really turning up the heat on this one.
If, and when, she is financially independent, she can do things like stay with her boyfriend. However, since she depends on you for everything, even clothes (really, dad? you pay for her clothes at 19! awesome), food, and a cell phone, explain to her that she is not different from your average 13 year old girl. Even though she is chronologically 19, emotionally and developmentally she is behaving like an underage tween here. She is old enough to grasp this concept. It's just that, when you are used to having had it all, it is easy to take things for granted.
Gee, I wish my dad paid for a car (AND insurance!), cell, and clothes ON TOP OF tuition when I was 19! Your daughter has it easier than many of us ever did.