I don't mean to pry, but hear me out.
If she is your girlfriend then you obviously loved her at one time. As you stated she loves you, and you care enough not to want her to be hurt by you.
Perhaps instead of finding an "un-love" spell you should ask yourself why you don't feel that you love her anymore. Sometimes people become scared when a relationship is becoming serious, and they deny their love for the other person, when they very well could be "the one". I'm not going to go into anything about "free will" but if she is "the one" then you could very well be harming both of your futures by attempting to remove her love for you.
Stress can cause a lot of things to our emotions, so can depression. You may be going through something within yourself right now that caused your change of heart. I believe that you should explore these psychological possibilities before breaking up with her. Your desire not to hurt her...is love.
Almost every healthy relationship goes through at least one seperation/break up. The seperation "makes the heart grow fonder". Sometimes this works for the best, sometimes the two really do find happiness away from each other. So if you do feel that you need space right now, it may be best to tell her that. Having time to yourself is essential for personal growth, discovery, and removal of stress in your life. If you are going through a lot right now, it isn't wrong to ask to take a break. If during this break you still feel that your love for her is gone, then you should be capable of looking her in the eyes and telling her so. If you do not love her, that shouldn't be difficult.
I would recommend that you perform a spell or ritual of clarification or truth. Something to make your mind and emotions clear to yourself, so that you can sort through this situation from within. That way, if there is love there (which I'm betting on) you may find the root of why you have trouble feeling it.
Once you understand yourself, you should be able to speak to her about it. If her behavior has caused your heart to retreat, then you should give her the opportunity to change that. If she truly loves you, she will want to.
At this point, a sweetening spell may be in order, to prevent her from taking offense and give a bit of insurance toward a peaceful conversation.
If things become resolved, you can cast a love spark spell, which is used to rekindle old love, causing you to rediscover the passion and compassion between you. If she is interested in magick she can even be involved in the spell, and it can be performed mutually.
It's never a good idea to force away love. Love is something that is beyond philosophers, psychologist, doctors, and scientists. And yes, even beyond witches! Love is the strongest energy a human being can produce. If it is beyond our understanding then we should not take it upon ourselves to remove it from anyone. If she loves you forever that is her right!
If you succeeded you could very well put her into a terrible state of depression. Suicide is a real risk at that point. She would stop loving you, but at what cost? There is always a cost.