Kit Harington had "lost his way" before checking into rehab.
The Game of Thrones star checked into a treatment facility last month after the ending of the hit HBO show - on which he starred as Jon Snow - "really hit him hard", and now a costume designer for the programme has said it was clear leaving the show behind affected him, because he was always a "sensitive soul".
Michele Clapton told Us Weekly magazine: "They were children when they started. And it became such an enormous thing, and I think when you stop, it's sort of a huge security blanket is taken away.
"I mean, they all became friends with each other, and then suddenly, there's nothing to go back to. So I would imagine that had something to do with it. He lost his way a bit.
"He was really a sensitive soul, and he was very upset in that final read through because I think it felt like home for [the cast], and they realised, 'This is it.'"
But the designer is confident Harington will "be okay". She added: "As much as he's talented and will go on to many other things, I think it's just after eight or nine years, it's a big adjustment. I'm sure he'll be OK, though."
Harington, 32, checked into Connecticut's Privé-Swiss wellness facility after self-referring himself to the centre last month to work on some "personal issues".
His representative said at the time: "Kit has decided to utilise this break in his schedule as an opportunity to spend some time at a wellness retreat to work on some personal issues."
And an insider later added: "The end of Game of Thrones really hit Kit hard... He realised 'This is it - this is the end', it was something they had all worked so hard on for so many years. He had a moment of, 'What next?'
"He's in the clinic predominantly for stress and exhaustion and also alcohol."