I'm a bit baffled if you can't see the difference between Amy pressuring Willow to use magick vs Cordelia convincing Buffy to go to a party.
First off, convincing someone to go to a party is a free choice. Buffy could 100% say no and the decision was autonomous. Yes, there is the peer pressure aspect of it, but autonomy was still present. Autonomy is the biggest factor here when comparing the two situations.
Amy 1) Forced Willow to use magick. Willow had no say, Amy literally "gave" her magick and 2) A drug addict is often under influence when making decisions concerning drugs, making them not autonomous. Essentially this means that Willow was not in position to make certain decisions and Amy was not in position to force magick on Willow. Amy directly violated Willow. Cordelia did not violate Buffy.
I'm not saying that Amy is worst or better than Cordelia, Amy was just as much under the influence as Willow was, making her actions sort of understandable, even if they were wrong. But Willow was right to cut Amy from her life. It's not even about forgiving Amy or hating her or loving her or whatever else, it's about what was healthiest for Willow.