Adam Driver says being discharged from the Marines was ''painful.''

The 'Girls' actor, who enlisted in the US army after the terrorist attacks in New York City on September 11, 2001, admits being forced to return home two years into his training after breaking his sternum on a mountain bike still ''f**kin' kills'' him, as he wanted to serve his country with his fellow recruits.

The 30-year-old star told the September issue of GQ magazine: ''It just seemed like a badass thing to do, to go and shoot machine guns and serve your country.

''To not get to go with that group of people I had been training with was...painful.''

The 'This Is Where I Leave You' star later set up the organisation, Arts in the Armed Forces, with his wife, Joanne Tucker, which deploys actors to entertain military bases.

He explained: ''Here's the thing: Life's s***ty, and we're all gonna die. You have friends, and they die. You have a disease, someone you care about has a disease, Wall Street people are scamming everyone, the poor get poorer, the rich get richer--that's what we're surrounded by all the time. We don't understand why we're here, no one's giving us an answer, religion is vague, your parents can't help because they're just people, and it's all terrible, and there's no meaning to anything. What a terrible thing to process! Every. Day. And then you go to sleep. But then sometimes, things can suspend themselves for like a minute, and then every once in a while there's something where you find a connection.''

Meanwhile, Adam, who is the son of devout Baptists, didn't tell his parents about his success on 'Girls' until after the second season.

He said: ''We have different views on the world. They have their life; I have mine.''