So, I want to make it clear that you should not be afraid that you will hurt her. If she doesn't like you, you can't hurt her. Therefore, since she likes you, she has to bear a large part of the risk of being hurt.
Your top priority is to explain to her as soon as possible. Long pain is better than short pain. Don't let her stay in deep trouble. This is the best way for her.
The way to refuse is to raise the other person and then list n reasons why you are not worthy of the other person. But it feels hypocritical. You better be sincere. I don't think you are suitable. You don't have the confidence and assurance to really love someone. I hope she finds her own happiness.
Don't ask someone to speak for her, it's disrespectful to her.