The actress and model was criticised for sharing the image on Twitter soon after the latest shootings at a US school, and told ITV1's Daybreak that she was "shattered" and "mortified" by the response in the press.
"It was just a completely out of context tweet, completely," Flanagan said.
"I did the tweet and it was some different pictures in one and it was relating to a hangover on Monday night.
"And I put something like, oh head... you know, and I treated this picture completely carelessly, like it was the stupidest thing I've ever done in my life."
She continued: "I started getting all these reactions of, 'How insensitive, how could you do this?' and I just felt sick.
"I just deleted it straight away, I rang my best friend and I was like, 'I can't believe what I've just done', and I said to her, 'Shall I reply to these saying how sorry I am?'
"And then I thought no, it will just draw attention to the fact of completely what I've done - it's so humiliating."
Asked if she could understand the reaction, Flanagan said: "Absolutely, I absolutely hated myself for it. I am incredibly sorry and I couldn't be more sorry.
"And I tweeted it completely out of context - it was just about this hangover and it was never, I mean I really do - my heart bleeds for all the families.
"I think what I did was ridiculously stupid… it was the stupidest thing I've ever done - I would never, ever want to hurt anyone."
She added: "I take responsibility for my actions, I shouldn't have tweeted that picture on Monday night even though it was referring to a hangover and I am so, so sorry.
"But I just want people to genuinely know how sorry I am and it really has affected me, I am absolutely devastated by it and I'm very, very sorry, especially if it's hurt any of the families."
Quizzed about why she posed with the gun in the first place, Flanagan said: "It was in October, seven weeks before this [the school shooting] happened.
"I sometimes forget - none of my friends are in the public eye and I kind of forget that I am in the public eye and I will take more responsibility and realise that I have to be more careful.
"I'm seen as a role model and I will definitely learn a lesson from this but I am truly, truly sorry."
Helen Flanagan's top I'm a Celebrity moments - photo gallery:
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