Although the couple had appeared to put Kristen's infidelity behind them - following her fling last year with married director Rupert Sanders - the relationship reportedly fell apart earlier this month because Robert felt Kristen had become ''too dependent'' on him.
A source told HollywoodLife.com: ''Through the years, Kristen definitely became too dependent on Rob saving the day and taking her back. He had to constantly feel in charge of Kristen and eventually it just became too tiring.''
Friends also believe Robert, 27, and 23-year-old Kristen are at different points in their lives right now.
The insider added: ''Kristen is too concerned about being a Los Angeles hipster and he's preoccupied with working these days.
Rob's ready to focus on his career and take it to the next level; Kristen's priorities obviously don't line up with that. He felt like an adult taking care of a child'' at times. Now he's glad to only have to be in charge of himself... He thinks she has a lot of growing up to do.''