Coming from someone else in a partially poly relationship, the biggest part of any relationship is honest and open communication and trust. The fact that 'K' didn't inform you of her entire feelings or intents and then expected you to deal with the issues after she went behind your back is really worrisome. I agree with 'treevoice' entirely when she said that 'K' was 'reckless at best'. Maybe you should take a few days off, if you can afford them, and spend some time doing self care? Then, after you have regained some energy, maybe you can confront 'K' with some healthy boundaries. I, for one, wouldn't agree to help her with her breakup feelings if she was starting to make me feel taken for granted and treating me dishonestly.

Also, judging from K's words of 'she knew 'this' would happen', it sounds like K already knew that she was polyamorous and chose not to tell you, rather than to have a fair and open conversation when you both were in a healthy and safe place. Springing sudden feelings for another girl and then being dishonest about them and then blaming you for not being fully accepting is frankly ridiculous. You are being very mature and it is important to be able to express your feelings to your partner, especially if they have been dishonest and you are trying to recover from that.

If you need someone to talk to, feel free to message me. I hope things get better.