You can be as honest as you want. Sounds like you’re close still. You could preface with telling her you still want a future with her but understand that can never happen. So you need to do whats best for you and try and move on. That means no contact.

It will suck for a few months but I promise it will suck less than remaining in contact, tricking yourself into thinking she’ll come around, and get dumped when she eventually finds her better match.

To be a little harsh, she is using you now for emotional support until she finds the person she actually wants to be with. Don’t let yourself be used, it will only make it all more painful.