How do I reply?

So recently, a girl who I always considered to be a friend confessed to me. I am a girl and mostly openly bisexual, but this is the first time a girl has told me she likes me. But here's the problem: one of the main reasons I started getting close to her was because I wanted to get close to a my crush from a while back, and they happened to be really close friends (I know it seems bad and I do regret getting close to her in that way). However, I have a bad tendency to flirt with the people I'm comfortable with, her included, and now I feel like this resulted in her liking me. What's more, she confessed in a hurry, and so I agreed to go on a date with her without thinking, but I don't really have those kind of feelings for her.

What do I do? I'm scared that if I reject her now, she's going to hate me and we won't go back to being friends, but I'm also scared to betray her by agreeing to go out with her and then hurting her feelings because I don't like her back.