It seems somewhat overdramatic to throw in the ex with a knife. Of course the ex being such an excessive scumbag does explain why Io was traumatised beyond what the scars alone would have done to her. Naturally such scarring would affect different people differently psychologically, depending on their circumstances. Perhaps Io was originally on the resilient side, but the bullying and the only one who "helped" her being actually a psycho wanting to exploit her would have done extra damage. I'd say she was resilient because she still kept interacting with this much better class, could start dating Kenshi, and even at the end she took the lead when Kenshi was being overly considerate. So, she wasn't mentally weak by any means. In that light having that knife scene served the purpose of showing how much she had to deal with in the past.