The former Fall Out Boy rocker - who announced he and Ashlee would be ending their marriage of two years earlier this month - became suspicious of her constant partying in recent months and he felt he could no longer put his faith in her.
A close pal said: "They have been talking about splitting up forever. Ashlee has been out late partying for the past few months... he felt he couldn't trust her."
On some occasions, 26-year-old Simpson would reportedly "lose track of time" and turn up at home at sunrise.
The friend added to US Weekly magazine: "He started getting burnt out. He would constantly check in on Ashlee and he'd have his friends call the house and her cell to make sure she said she was where she said she was going to be."
Ashlee - who has two-year-old son Bronx-Mowgli with 31-year-old Pete - is now believed to be staying with her sister Jessica Simpson while she deals with the breakdown of her marriage.
However, Pete is said to been in tears about the end of their relationship.
A source said: "Pete has been crying a lot to her about this. He is crushed, and does not want it to be over."