The 19-year-old called time on their relationship amid rumors that father-of-three Disick, 34, was cheating on her.
A source revealed: "She couldn't take it anymore.
"He goes in waves of partying really hard and then will have his good days and it's just very toxic behavior."
Disick was reportedly telling people that he was single while at Kanye West's Wyoming listening party.
There were even reports that he was getting close to a mystery girl at the event.
The source said: "Sofia broke up with Scott after the pictures and rumors of his behavior in Wyoming were floating around.
"She couldn't take it anymore. Scott has been off the rails recently and is not in a good place.
"He goes in waves of partying really hard and then will have his good days and it's just very toxic behavior. It's very up and down."
RadarOnline.com previously reported how Disick had previously hit rock bottom after partying too hard with his teen girlfriend in the past.
We pay for juicy info! Do you have a story for RadarOnline.com? Email us at firstname.lastname@example.org, or call us at (866) ON-RADAR (667-2327) any time, day or night.