The show's May 22 season finale will see several characters graduate from McKinley High.
Morrison, who plays Will Shuester in the Fox series, told Next Movie: "It was incredibly sad. Even though I knew they were graduating, I wasn't prepared to be as emotional as I was.
"We've all been crying for the last week. It's hard saying goodbye."
Morrison can next be seen in ensemble comedy What To Expect When You're Expecting, in which he plays the love interest of Cameron Diaz's Jules.
"Cam and I both play celebrities," Morrison said. "It was really important to give them some humanity, and to show that even if you're a celebrity, it doesn't matter who you are when it comes to making difficult decisions. It's part of being human."
What To Expect When You're Expecting is now playing in the US and will arrive in UK cinemas on May 25.
Watch the trailer for What To Expect When You're Expecting below: