Hey. Seems like you've got yourself into a pretty messy situation eh? I think for future reference don't say yes unless you want it!
When you see her tomorrow I think that you need to be upfront and tell her out straight that you just want to be friends, the soon you get this out of the way, the more she'll respect you. Its unfair for you to carry on in this relationship if you don't like her, so the sooner you get it over with the better.
You might be thinking 'oh I don't wanna hurt her feelings though', but do you really think you staying in a relationship you don't want is going to make her feel good about herself? Of course it isn't! Its best for both of you if you get this out of the way and dusted under the carpet.
Of course she might be angry with you, but you have to look at it from her perspective, after all you did say yes when you didn't really like her. I'm sure both of you will work things out in the end, but you really need to get this sorted, tomorrow!
Good luck, PM to talk some more.
~ Sophie