First of all, stop belittling yourself and calling yourself stupid. This isn't a situation that you asked for or clearly want to be in, but stuff like this happens. It's happening to you, it's happened to me, it's happened to a fair number of people in this thread, I'm sure. Like many people have said, you can't help that you don't like your friend that way and your friend can't help that she likes you. My advice is that if she really does like you like she says, she'll respect your feelings and she'll be your friend even if you don't date her. If she doesn't respect the fact that you're not interested in dating girls, she's not a very good friend.
Don't feel bad for your feelings or lack of feelings. This is a part of life and stuff like this happens a lot. There's going to be lots of time in your life where someone likes you and you don't like them back, and vice versa. Just don't settle. Don't tell yourself "This person likes me... no one else does... maybe I should just date them." That's not good, that's not a healthy outlook. Don't settle for someone you don't really care about in that way. Basically, don't date your friend out of pity. It'll just make both of you miserable.
Don't feel bad for your feelings or lack of feelings. This is a part of life and stuff like this happens a lot. There's going to be lots of time in your life where someone likes you and you don't like them back, and vice versa. Just don't settle. Don't tell yourself "This person likes me... no one else does... maybe I should just date them." That's not good, that's not a healthy outlook. Don't settle for someone you don't really care about in that way. Basically, don't date your friend out of pity. It'll just make both of you miserable.