After years of romantic comedies, Drew Barrymore is tapping into difficult places in her psyche for her next role.

"I was excited to bring my own pain to something," Barrymore, 34, who plays "Little Edie" Beal in HBO's Grey Gardens airing in April, tells W magazine. "I mean, contrary to my happy-go-luckiness, I have so much darkness in there."

To prepare for playing Beal, a former society beauty with acting dreams who ended up living in squalor with only her overbearing mother, Barrymore drew on memories. "I got institutionalized as a kid, and I felt like I was back there," she says. "A lot of times I found myself unhappy and isolated, and the only other time I felt that way was when I was in there."

Barrymore found it especially liberating to be able to focus on herself for a change. "I like making people feel good, but it was great not to have to please anybody," she says. "I was out there for myself and for her."

The also opens up about her five-year relationship with The Strokes' drummer Fabrizio Moretti, calling it "one of the most, if not the most, important relationships I've had in my life. We're still very close."