Tell her (to her face) that you realised a while ago that your feelings towards her started to change, you've given it time in the hope it was just something you were going through (which you likely have), but despite that, those feelings have never returned and now you don't see a future together.
Key here is don't fuck off straight after, talk to her, but don't be lured back in if she starts to get upset, stick to your decision, unless you actually believe she will make the changes she says she will and you're prepared to try again.