After their breakup, the Keeping Up with the Kardashians star, 18, is "absolutely fuming," a source close to Jenner says.
Though it's unclear what exactly went down between the duo – or when – the source tells PEOPLE that Tyga "broke her trust."
"She's caught him lying and talking to other girls behind her back before, and this was the last straw," the source says, adding that Jenner "feels super betrayed and is just so pissed off, damaged and hurt about it."
"She plans to erase him from her memory and act like he never even existed in the first place," the source continues.
Tyga, who turned 26 on Thursday, celebrated with friends that evening in Los Angeles – but Jenner, as well the rest of her famous family, was notably missing from the festivities, even though she flew back from Australia that day.
GVK / Bauer-Griffin / GC
An onlooker told PEOPLE that Tyga spent much of the night behind the DJ booth "hanging with Sam [Ronson] and spinning tracks with her."
The Kardashian/Jenner women also did not share any happy birthday messages for Tyga on social media.
Just hours before news of the split broke Friday, Jenner shared a glamorous picture of herself on Instagram.
Tyga, meanwhile, has also not commented on the breakup but continues to promote his new single, "Happy Birthday," on Twitter and Instagram.
The stars first sparked rumors of romance last fall. Tyga confirmed they were dating in March, and after Jenner turned 18 in August, they went public with their love.
In the months since, Jenner and Tyga stepped out together in both Los Angeles and New York City, showed plenty of PDA on Snapchat and vacationed together in Mexico and St. Barts. She even appeared in two of his music videos, though she has said she values keeping their relationship private.
I do think that if you let people in too deep, it gets hard – your fans get too invested, and then they know when you break up and make up," she told Elle Canada for the magazine's December issue. "I don't want people to know every time we fight or break up."