"I've set high standards and ...I don't think I had a terrible championship but I can play a lot better," Beckham said on Monday before England's friendly against Ukraine at St James' Park, Newcastle on Wednesday.
"I know I've been given a talent which can't disappear just like that. For four games I didn't perform like I can. I want to prove that's right."
"I had done too many weights at the end of the season and didn't realise it had affected me as it had and that's not an excuse," he said.
"I was a little bit bored after training and ... I would go into the gym for an hour and work out but I've stopped doing weights now and I've lost weight and I feel good."
The Real Madrid midfielder said he had been affected by the criticism after England's quarter-final loss to Portugal.
"I've been criticised over the years for endorsements but never before for my on-field play which upsets me, that's why I sat down with my family to discuss things," he said.
"There were a few doubts put in my mind but I never doubted myself (and) ... after Euro 2004 I spent time and rested with my family, and just took three weeks off and did absolutely nothing."
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"We were all disappointed in the summer because we felt we could have gone further but we've got to look at the World Cup," he said.
"You've got to accept the pressure and that people might criticise you if things aren't going well, but I'm strong enough to take that.
"The pressure will always be there at this level both on and off the field."