Cheating on him when he added her to his insurance plan and was paying for her to be able to have access to the healthcare she needed due to her diabetes after she had lost her insurance when Derek was never ever obligated to do that for her! He did it out of love and care. Derek showed his love in different ways instead of ever verbally expressing it and that wasn’t ever enough for her. She shoulda known how important she was to him when he added her to his insurance but instead she just picked at him over every little thing trying to cause all of these unnecessary fights! She’s so toxic. Derek deserves the world and I believe he will find the right girl for him who will show him the appreciation he deserves and actually supports his dreams - no matter how big or small they might be.