Tell her yourself. You must not let her find out through anyone else. Chances are, she is going to be upset (unless, by chance she feels the same way, but don't count on it). If she learns of your feelings through someone else, you risk embarrassment on top of hurt on her part.
Meet in a neutral location where you can both be comfortable to speak your mind. Restaurants or other crowded, public areas are not good choices because you both feel restricted and unable to speak or react in ways that reflect your true feelings. Sure, it may be easier for you to stick her somewhere she can't cause a scene, but she deserves better than that.
Speak the words out loud and in person. A text message or email may be easier for you, but she is guaranteed to be angrier with you if you choose a cowardly route.
Cover all your bases. Tell her exactly how you feel and leave no stones unturned. Say things like "I wanted this to work," "I tried to have feelings for you, but I can't force it" or "I wish I could love you, but I can't get over what you did to me."
Prepare to be asked questions like "Why," "What can I do to change your mind," and "Can you give it one more chance?" You can't blame her for trying, but do be firm and do not waver.
End the relationship with a clean break if you want nothing to do with her anymore. You must not say "We can still be friends" unless you truly want this, but be prepared for a long, awkward road that often ends with a broken friendship.
- Slip her a copy of Greg Behrendt and Liz Tuccillo's "He's Just Not That into You" if all else fails or you can't bring yourself to break her heart. It's worked wonders for thousands of other girls. It just might be the passive-aggressive wake up call she needs.