The Fall Out Boy rocker - who split from the 28-year-old singer in February 2011 after two and a half years of marriage - blames their divorce on his addiction to prescription drugs, including Xanax and Klonopin, and credits their four-year-old son Bronx for giving him a reason to seek help.
The 33-year-old bassist told the new issue of Rolling Stone magazine: ''I was probably physically and mentally addicted. It started from insomnia and anxiety from flying, then it spiralled.''
Pete, who recently called Ashlee an ''awesome'' mother, admits he had previously abused the drugs but it ''tapered off'' when his son was born in 2008.
He suffered a bad relapse in 2009 when his band went on hiatus but his problems got even worse after his split from Ashlee.
He said: ''I felt like a loser already. I'd basically gone from being the guy in Fall Out Boy to being the guy who, like, hangs out all day. I didn't see how I'd ever come out on the other side.''
''I was scared of everything. I thought there were always people listening to me. Like, I had my house searched for bugs. It was crazy.''
After his divorce was finalised, Pete sought professional help for his issues and credits his son for helping him to stay sober.
He said: ''You're up in the morning, and you have to really be there. It's not like getting up for a radio interview. In some ways, that was super helpful for my soul.''