Lea, 26, was left devastated last month when 31-year-old Cory was found dead in a hotel room in Vancouver, Canada, after suffering an alcohol and heroin overdose.
But yesterday, the actress bravely joined her fellow Glee cast mates back on set and began filming new episodes of the hit musical show.
Referring to her character, Lea told fans on Twitter: "First day of Glee today..And at my first wardrobe fitting for Rachel Berry for Season Five! I've missed her so much❤"
Earlier this week, Lea used the social networking site to thank fans for "helping her through" this awful period in her life.
Sharing a picture of herself and Cory, Lea said: "Thank you all for helping me through this time with your enormous love & support. Cory will forever be in my heart."
Following Cory's death, Glee creator Ryan Murphy admitted that they would have ended the show if Lea felt she could not continue.
But he added: "Ultimately, everyone involved made the decision, 'OK, what does Lea want to do, what would Lea feel comfortable doing?' This is what she wants to do."
When Glee returns to TV screens in the US next month, it will feature a special episode that will pay tribute to Cory.