Sean Spicer, who quit as the White House press secretary after just six months, said he made several ‘missteps’ during his time in the role.
Mr Spicer was widely mocked after he insisted that Trump pulled the largest crowd ever on the National Mall in Washington DC – despite pictures telling a different story.
Speaking to BBC’s Newsnight, Mr Spicer said he would love a ‘do-over’ of that day.
At the time, Spicer had insisted that Trump drew ‘the largest audience ever to witness an inauguration, period, both in person and around the globe’.
He told the BBC: ‘I wanted to change the focus to the number of people who had watched it, not just on the National Mall, and I clearly didn’t do a good job of that.
‘That goes down as one of the days I’d like a do-over.
‘Nobody was happy with me that day.’
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Mr Spicer also spoke about when he said Adolf Hitler had not used chemical weapons on his people during the Second World War.
Admitting the ‘misstep’, he told ITV: ‘It’s painful sometimes to think of the missteps I may have made that made other people reflect poorly on the country that I love.
‘There was a level of intensity where you were always five reporters ahead, what was coming, and I had answered the other questions on message.’
Mr Spicer often battled with journalists during briefings, and he admitted to feeling ‘constantly’ under attack throughout the presidential campaign and at the start of Trump’s time in office.
He said: ‘There were days in that job that were extremely lonely because no one was happy with me.’
Despite his short time in the role and difficulties during his tenure, Mr Spicer said it was ‘an honour and a privilege’ to have the job.
He added: ‘When I was a child I didn’t think I would get a tour of the White House, to work there was an honour.
‘I knew I was becoming the story too often.
‘Time after time, I became the focus, and a spokesman should be speaking for someone else.’