One of the most emotionally painful things most people will go through (other then death or major illnesses/accidents) is to get dumped. Especially if they do not see it coming.
So to use a 'line' on her is pretty insensitive, IMO.
And if you want serious advice, how can I give that? I have no idea why you want to end it.
But if you want out, tell her as quickly and as honestly as possible.
It is times like this that, I think, decide how strong of a person you really are. It's hard to do something you know will hurt someone you care about. But the faster you end it; the faster she can move on to someone who will want her more then you do.
Ending the relationship is the best thing you could do for her. And if you do it with respect and some honour - then I think you are doing her a big favour.
Sorry to sound preachy, but this woman is about to (probably) get her feelings very badly hurt. How you handle it can make it a WHOLE lot better for her.