07/25/2008 at 05:00 PM EDT
may have been surrounded by adults dressed as superheroes and Star Wars
characters at San Deigo's Comic-Con International, but the heiress-turned-actress says "I just feel like a grown-up now," thanks to her current romance with musician Benji Madden and their more low-key lifestyle.
"I think my whole life I was kind of living as a teenager and not really taking responsibility," Hilton, who visited the Con to support her new film – and well-received performance in – Repo! The Genetic Opera
, told PEOPLE. "Now I realize that I'm an adult and I'm running a huge company and I'm in love. I'm in a great relationship. I have my family. I'm just excited for life."
When the notion of tying the knot with Madden (whose brother Joel is the paramour of Nicole Richie
) was broached, Hilton was cautiously optimistic. "I don't know," she mused. "We're so in love and so happy right now and it's only been like six months. So we'll see what happens, but we're so in love and so happy and things have never been better. I finally have a nice guy."
The couple is also making beautiful music together in the studio. "We just wrote together," Hilton said. "I had a whole recording studio in my house and so we've been working there every night. We wrote together on two songs for my Best Friends
"He helps me because he's an incredible writer and we're on the same wavelength and so we're working well together as a team," she said. "Benji will bring us beats and then we literally just sit there and write to the beats about what I want to talk about. The music is really fun. It's really just happy and fun dance music."
And Hilton and Madden are also occasionally pulling babysitter duty for his niece Harlow. "We just saw her like a week ago. She had her six-month birthday. I can't believe it. She's getting so big and is so cute. She's laughing now and trying to call which is so sweet. I love babies!"