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 so she could continue getting her insulin. Derek showed his love & affection in different ways instead of ever verbally expressing it and that wasn’t ever enough for her for whatever reason despite the fact her husband had a heart of gold and always put her and her needs first. 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 over the tiniest and stupidest things - like his poem for example! Just because he’s never experienced love before doesn’t mean he wasn’t ever a teenager who experience strong lust for someone before and Derek is such a dreamer that I believe him when he says just because he’s never experienced it doesn’t mean that he can’t dream over what he thinks love should look like. And even if he did “fall in love” and wrote that poem because of it that was from years ago when he was in high school and it has no effect on their relationship so I didn’t quite understand why she was making such a big deal out of it when he never did anything in their marriage to ever make him suspicious of cheating or not putting Katie first???? When it turns out she was the one cheating! Talk about projecting much… 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.