There area couple of things you could do:
1) Ashlee could be cheating on him with his brother
2) Ashlee could be pregnant with someone else's kid
3) Pete got tired of Ashlee's drug or alcohol problem (obviously not in real life lol..but for story purposes)
4) Ashlee's father interferes too much in the relationship and Pete's tired of it
5) Ashlee was using Pete to better her career
I know, all of these are her fault....didn't know how you wanted your story to go lol..i'm sure i could think of some where it would be Pete's fault lol
I'm excited to see what happens...i like all your other stories :-)
Author's response
Wow! Lots of ideas! Thanks! And also thanks about the stories, I love to know I have fans of my work.