Photographed July 17 in a steamy makeout session with her Snow White and the Huntsman director Rupert Sanders, Kristen Stewart sure kept a poker face when hiding the tryst from boyfriend Robert Pattinson.

As the new issue of Us Weekly reports, Stewart, 22, and Pattinson's romance of more than three years seemed stronger than ever in recent months. At least from the point of view of Pattinson, 26, who began making plans to propose to Stewart and "spend the rest of his life with her," according to a pal.

PHOTOS: Rob and Kristen's sexy relationship

The Twilight actor had even encouraged his girlfriend to be more open about their love, and while Stewart initially resisted, she gushed to a fellow attendee at a June 23 wedding in New Jersey that she was anxious to have a baby with Pattinson.

In the days following her photographed tryst with married father of two Sanders, 41, Stewart went about her business as usual -- with Pattinson still on her arm.

The couple seemed especially cozy during a July 19 date at Hotel Cafe -- a mere 48 hours after Stewart's fling. During their outing in Los Angeles, "she leaned against him," says a witness, who adds that Stewart and Pattinson's PDA was "cute." Two days later, they made an appearance at the Teen Choice Awards in L.A.

VIDEO: Rob and Kristen's special bond

Once Us Weekly released a set of photos of Stewart with Sanders on July 24, Stewart was forced to come clean. "I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected," she said Wednesday.

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Stewart also addressed her longtime love by name in her apology. "This momentary indiscretion has jeopardized the most important thing in my life, the person I love and respect the most, Rob," she continued. "I love him, I love him, I'm so sorry."

This article originally appeared on How Kristen Stewart Hid Her Steamy Affair From Robert Pattinson