Most Helpful Girl
There's no way that it will not hurt her so just know that she will feel pain and that it's not possible to avoid that. But also keep in mind that you're doing the right thing and that in the long run she'll be very happy that you told her and didn't keep it up for longer. The less time she lives in her illusion that you two are happy, the better. Just call her up, meet her in person and then tell her the truth. It will be tough. It will be one of the most difficult things you've done but just know that it's normal to lose feelings and that it's not your fault. If she has anything to say, listen. If she gets angry, then accept it and don't just walk away. Try to talk to her and offer her your friendship. Lastly do not completely abandon her after the break up... oh gosh it's awful to break up with someone and for them to just walk away and never hear from them again. My last ex dumped me in a very painful manner and when I simply asked him a question over text he blocked me from whatsapp. I just asked him if he wanted pictures that were taken on a night out... it really tore me apart that someone that meant so much could be so cold. So please... just be nice to her afterwards and after a week or so just check in and ask her if she's still OK. It's just comforting to know that someone you just spent the last x amount of months or years with still cares about you.
Most Helpful Guy
Be honest and up front. It is gonna hurt but the sooner you do it the better