How can I get rid of my "friend"? She's making it near impossible!?

[Sota]

I've been told the nicest way to get rid of a friend you no longer like is to gradually avoid them more and more, and make excuses not to meet up with them. I really, really don't like her. I can't say I didn't try, I really did and I'm not a b*tch. I met up with her a few times even after I decided I didn't like her, she is constantly patronising me and putting me down. We are just on different wavelengths I feel. I have just started making excuses to almost everything or saying something vague like "oh sorry I can't make it then, maybe another time!"... I've had to start saying at least "maybe another time" because I've put her off so often now, it's looking just plain rude. I kind of hoped if I no longer asked to meet up with her and just kept making excuses she would eventually give up, but she keeps on asking me to do something. But I think she knows something's up because the way she asks is getting progressively more and more b*tchy.
It's stressing me out a lot. I don't want to openly talk with her about it, really not. I don't feel it's worth it. She hasn't done anything "unforgivable", but she genuinely leaves me feeling irritated and a little upset every time we meet. I'm tired of it but I don't know the nicest way to get rid of her. These are all via facebook messages btw.
also just want to add, we used to be very close in high school, but 3 years on she's a different person. she was always kind of opinionated but now it's just gone over board.