There IS no magic solution to this that no one else has thought of. There's no back door. However...
My advice would be to tell her that there's something you've been scared to talk to her about because you've been terrified about hurting her feelings.
Say that you feel you have to talk to her about it NOW before things go any further, because the longer it goes on, the harder it will become for you to tell her.
Say that when you first got involved with her you were swept up by the whole affair, but as time has passed you have come to realise that things aren't going to work out between you, so you feel you should end things now before it becomes too hard.
You're just too different.
You want different things.
(Whatever you think fits).
You can probably take it from here.
One thing I can say is that once this conversation is under way, and there's no going back, you'll start to feel relieved that you are actually doing something about it. You will also find that however hurt she might or might not be, life will go on! It's not the end of the world - just do it.
Alternatively, if you can't think of you own excuses, you could try this [url=http://www.ineedsomespace.com/love/love_solutions/letters/diy_letter.html]"do it yourself" multiple choice breakup letter[/url].
I Need Some Space!
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