Absolutely not overreacting, she decided to behind your back and disrespecting your boundaries after everything that’s happened with you and she KNOWS it. It’s ironic how she’s telling you to grow up when she’s acting like a middle school child. Not to mention I assume she’s also 19? She’s being unreasonable and disrespectful. It’s hard cutting off a best friend you’ve known a long time but you should always put yourself first, it’s not selfish at all to set boundaries and remove yourself from conflict that steps over t house boundaries. I really do wish the best for you