The 'Twilight' co-stars have reunited after he forgave her for cheating on him with 'Snow White and the Huntsman' director Rupert Sanders, but Robert is being careful to take things slowly.
A source told RadarOnline.com: "Kristen is working hard to try and force the relationship into what it was before.
"But the truth is, she's going to have to take it slowly with Robert because he's still having trouble trusting her again.
"For him to carry on like nothing happened is very hard. Kristen is eternally sorry for cheating on him and he has forgiven her - kind of."
While Kristen, 22, is adamant her relationship with married Rupert was never sexual, Robert, 26, is having a hard time believing her.
The source added: "The problem is he can't forget her infidelity. Kristen claims she never actually slept with Rupert, but Robert still has that nagging feeling of doubt that she did.
"It's too early for him to fully accept that they're an item again, but Kristen is pushing for them to go back to how they were."
The pair will start promoting the last 'Twilight' film, 'Breaking Dawn Part 2' together soon, but film company Summit Entertainment isn't taking any risks with their reconciliation and booking them separate hotel rooms.
A source said: "Summit isn't taking any chances that the couple will have problems again and be forced to scramble at the last minute to book one of them a posh hotel suite.
"In fact, in several cities, rooms have been booked for Kristen at a separate hotel from the rest of the A-list cast. Rob and Kristen are always professional, but the fact is she cheated on him, and this media tour needs to go off without a hitch."