"Every day is so hard. Just getting up in the morning means remembering that Cory is gone," a source close to Monteith's girlfriend and Glee costar tells PEOPLE. "It's incredibly painful."
Michele, 26, returned to the set of the FOX hit earlier this week, but life without Monteith – who ingested a fatal mixture of alcohol and heroin on July 13 – continues to prove difficult.
"It's a struggle. She is incredibly strong, but that doesn't make it easier," adds the source.
As the group reconvenes for its fifth season, Michele is expected to make her first public appearance at Sunday's Teen Choice Awards, where Glee has five nominations. A source confirms to PEOPLE that the "entire cast" will be in attendance for a planned tribute to their late costar.
Fans of Monteith may make donations in his name to three charities that were especially important to him: Project Limelight Society, which exposes youth living in Vancouver's Downtown Eastside to the arts; Virgin Unite, founded to support entrepreneurial efforts to better the world; and