Why is this

My husband and i have been separated for almost 5 months now. I have my own place, we barely interact outside of discussing our children, and i started paperwork to file for divorce. Then i found some pictures of some girl in his email (why i was there is a long story) but i found them and they killed me i was so angry. he cheated while were in our marriage together i don't know why i believed he would change but he didn't and i don't know why i was so hurt?