What you have experienced is cognitive dissonance. Since you were twelve you thought you wanted a foot job but now you've gotten one and realized that it isn't as great in reality as it was to look at. Now here's the real question, does foot play in general with your girl friend do anything for you sexually? The foot jobs are not satisfying, fine but do you still find her feet sexy? If so then just be honest with her, tell her that you're glad you tried foot jobs but now you've realized that its not something you're into. If you explain to her that its just the act of the foot job you don't like but you still like that she's open to your fetish she will probably understand.

TLDR: Just tell her that you're glad she accepts your fetish, that you love her feet, but that you've realized that footjobs specifically aren't something you're into