It's a combination of being immune to him after so long (they were together all the time 24/7), her will breaking his hold after he asked her to take a life and him being hit by the bus I'd imagine. The easy answer is 'power of love', but the truth is more complex. The flashback to the rooftop shows that she was subtly defying him long before that at any rate. And he was growing tired of controlling her vs her choosing to be with him.
Heck, doesn't he even say she was slower to figure out than he was?
( How that made her "immune" from that point forward, and why the whole subplot with a vaccine was entirely pointless, is another debate... )
The vaccine stuff was subtly done. Kilgrave's Dad sprays himself with it, it doesn't work, and they never mention it again. They could've sorted it, but the rushed deadline killed that idea.