The former Fall Out Boy star is said to be "crushed" about the end of his marriage to the 26-year-old singer, who is wants to move one from their two year marriage.
A source told RadarOnline.com: "Pete has been crying a lot to her about this. He is crushed, and does not want it to be over.
"Ashlee is done, she loves him, but she`s ready to move on. When he travelled for work, she had a lot of time to think and in the end just felt happier without him."
Ashlee is now keen to focus on their son, three-year-old Bronx Mowgli and her career.
The source added: "They married really young and have grown apart, she just wants to focus on her child and her career."
The pair filed for divorce at the Los Angeles County Court citing "irreconcilable differences" but insist they remain friends.
In a joint statement, they said: "After careful consideration, we have made the difficult decision to file for divorce.
"We remain friends and deeply committed and loving parents to our son Bronx, whose happiness and well-being remains our number one priority. We ask that everyone honour our privacy as we navigate this next phase of our lives."