The Glee star's boyfriend of 14 months and co-star was found dead in his hotel room in Vancouver, Canada, on July 13 due to an overdose from heroin and alcohol, and despite bravely returning to the set of the FOX series this week, Lea is struggling to get out of bed every morning.
A source close to the brunette beauty told People.com: "Every day is so hard. Just getting up in the morning means remembering that Cory is gone. It's incredibly painful.
"It's a struggle. She is incredibly strong, but that doesn't make it easier."
'He was very special to me'
The actress made her first public appearance since Cory's sudden death at the Teen Choice Awards in Los Angeles on Sunday, paying an emotional tribute to her late boyfriend and co-star.
"I just wanted to be here today to personally thank all of you and tell everyone out there how much all of your love and support has meant to me over these very difficult past few weeks," she said as she took to the stage to accept her award for Choice TV Actress in a comedy for her role on Glee.
She dedicated the award to Monteith saying, "For all of you out there who loved and admired Cory as much as I did, I promise that with your love we're gonna get through this together. He was very special to me and also to the world."
Earlier this week, Lea said it felt good to be back at work on the set of Glee so soon after her boyfriend's death.
She posted a picture of herself in character, Rachel Berry, sitting on the bed, gazing at the floor, with the caption: "Feels so good to be home..."
She also wrote: "First day of glee today. And at my first wardrobe fitting for Rachel Berry for Season Five! I've missed her so much (sic)."
Glee bosses will film a memorial episode for Cory - who portrayed Finn Hudson - which will reference the dangers of substance misuse.