The on/off couple have recently been reunited after the actor returned from a 2-month trip filming `The Rover` in Australia, but reports claim he still has his doubts.
It is said Robert, 26, is still "scared" after Kristen, 22, had a fling with `Snow White and the Huntsman` director Rupert Sanders last summer.
A source told RadarOnline.com: "He`s still very cautious when it comes to fully committing himself to her, and she`s being really clingy right now. He`s a little scared that Kristen could cheat on him again.
"So, just when it looks like all is going swell, his crippling doubts come into play and he questions if he`s doing the right thing.
"Robert proposed the idea that they don`t necessarily have to live in each other`s pockets, talking about how much he missed her while he was away and said that made him realize he does still want her in his life.
"Absence makes the heart grow fonder though, and he feels their relationship will be ultimately stronger if they`re not always together."
Although the `Remember Me` heart throb seems to still want to take things slow, the couple have been inseparable recently and he was grateful for the "effort" Kristen made when he came back from Australia.
The insider added: "[She] wants to spend every minute with Robert, who has now managed to get over his reservations about living back in Los Angeles.
"She`s even talked about attempting to take summer off to spend every day with him.
"He was happy that Kristen made such an effort after he returned to the States; she rallied all their friends together for a big welcome back party and Robert was touched by the gesture."