"No one likes being criticised. It sucks," Dunham admitted, before the sixth and final season of her acclaimed TV series begins this tonight (8.30pm, Soho).
"I'm glad for the dialogue the show created around issues of representation and issues of feminism,but no one likes being constantly called out. Even if you don't think it's affecting you - like I really don't care if a guy in the midwest calls me fat, or a Republican tells me what an idiot I am - you can't have that much negative energy directed at you without there being some cost to your emotional and personal life. I am public about that because I think it's important to say."
Dunham hopes her comments will help bullied teenagers feel they are not alone. "I'm not public about this because I want to say 'Oh boo hoo, treat celebrities like people', but because there are a lot of teenagers who are bullied on the internet all day every day and it can lead to suicide.
"I'm someone with a lot of resources around me. I have a yoga teacher and a therapist. I'm taking great care of myself and it hurts even me, so what does it do to a teenager in Florida when her entire class turns against her on Twitter or passes photos of her around online? I don't really care what happens to celebrities on the internet; I care what happens to people on the internet."
Her close friend Jemima Kirke, who plays the bohemian Jessa in the show, said that Dunham has struggled with being in the spotlight. "When we were at school, Lena was friends with so many different groups of kids and no one hated her ever," she said. "When that changed after Girls, it was really hard on her, because she had always valued that she could really understand people and they understood her."
Jenni Konner, co-writer and executive producer on Girls, agreed. "I actually think Lena's very good at hearing considered and thoughtful criticisms," she says. "She will change her mind and explain her process and talk about why she's apologising -which I think is rare because so many celebrities dig in and don't own their mistakes. She's a good role model in that way."
And, for all her concerns, Dunham has no intention of shying away from controversial material in the final season. The third episode in particular is a sharply written, brutal examination of creativity, privilege and misogyny, in which Dunham's Hannah, now making her way as a journalist, faces off against Matthew Rhys's successful writer, a man who has been accused of abusing his position of power with young women.
"That episode comes from a really personal place," she said. "I really wanted to write about the kind of stories I was hearing from young women about relationships they thought were mentorships - but things suddenly went south, and it was very painful and caused them to question themselves. I wasn't looking to give an answer or create a moral resolution. I just wanted to ask the questions because this was on my mind: the way people abuse positions of power and creative capital to get what they need from others and how destructive and traumatic that can be."
Girls itself has often been criticised for being about self-indulgent, solipsistic white women (something Dunham would argue was precisely the point: her protagonists are frequently there not so much to be identified with as laughed at) and many of the cast admit that because the writing is so realistic, it has been hard for people to consider them apart from the characters they play.
"We're so blended into our characters that people don't see us separately," said Kirke. "It's really problematic. For example, when Hannah has a line that's delusional and uninformed, that's taken as Lena being delusional and uninformed, and that's why so much criticism comes to the show."
Zosia Mamet, who plays the fast-talking, awkward Shoshanna, said that the problems with perception extend into auditions elsewhere. "I don't fear typecasting, [but] I experience it. People just forget that women are actors, so while men are allowed to play a vast variety of types and roles, we play one thing that's in the public eye, or a success, and people go 'Oh she's that'. That's definitely something we're all fighting pretty hard against."
Perhaps because of this pressure there's a clear sense that this was the right time for the show to end. "We'd told the story we needed to tell and it's nice to finish when you still have a passion for what you're doing," said Dunham. She won't be drawn on what fans can expect in the finale except to say that, despite her enthusiastic campaigning for Hillary Clinton, the issue of the new US president will not be addressed.
"Nobody needs any more pissed-off millennial white girls explaining what's wrong with Donald Trump," she said, adding that away from TV she remains committed to activism. "The organisation that's happening now is showing that protest matters, calling your representatives matters, becoming involved in community organisations matters, sending your donations every month matters. It has never mattered more to show up with your money, your body, your time and your voice than it does right now."
Who: Lena Dunham
What: Girls, the final season
When: Tonight, 8.30pm