OP, i was in a similar situation a few years back... really cute, smart, had her shyt together, tons of thirsty nikkas sweating her. i just didn't want to take the leap that she wanted to.
my suggestion--
if you've really decided that you're not feeling her, it's better to break up and
give her space than to lead her on. which means no post-break up sex, no 3 am "wyd" texts... women have an easier time finding someone to fill the void temporarily. you should also think about how invested you are in this girl. for me, when i broke up, i felt like a huge pressure was relieved from my chest, but i generally get mentally fatigued being in relationships

. also, no matter how nicely you put it, she's gonna cry a lot and get mad... don't be defensive and allow it to escalate.