I'm really frustrated, I want to break up with my girlfriend but I don't want to hurt her.
That sounds serious, why do you want to break up with your girlfriend?
I just don't love her anymore, but she loves me and she'll be devastated if I break up with her.
I know that you don't want to hurt your girlfriend, but if you don't want to be with her anymore then you should break up. I think you should tell her how you feel in a kind way, so as to let her down easy.
It will be very hard not to upset her, I'm not sure how to do it.
That sounds really tough, but ultimately you need to be happy. Breaking up will be painful for her, but if you don't love her there's no point staying in a relationship with her.