She doesn’t feel like she can trust you, and doesn’t think that trust can be repaired. She probably does think it will always be in the back of her mind. It’ll pop up whenever things aren’t great. You get distant - is he cheating? You come home late from work - is he cheating? You like another girl’s photo on social media - is he cheating? I’m sure it hurts but not everything is fixable. She is making the best decision for both of you.