The 28-year-old helped Gareth Southgate's side keep two clean sheets in those game but said he had doubts about his ability in the buildup to the matches.
"I did have a tough time in the lead-up to the opening game against Croatia," Mings told the Sun. "I think I'm a lot more hardened to outside influences now, but my mental health did plummet.
"And I have no shame in admitting that because there was so many unknowns about me going to that game.
"I was probably the only name on the teamsheet that people thought: 'Not sure about him.' And that was something I had to overcome.
"When 90-95% of your country are having doubts over you, it's very difficult to stop this intruding on your own thoughts. So I did a lot of work on that with my psychologist.
"I was given a lot of coping mechanisms -- whether it was breathing, meditation, or just learning how to bring yourself into the present moment. To stop letting your subconscious take over. It was hard. I didn't really sleep very well before that first game."
There has been a lot of spotlight around the well-being of athletes after U.S. gymnast Simone Biles withdrew from events at the Tokyo Olympics to focus on her mental health.
"It's just great that we are playing in a time now when you can speak about mental health, and how you are feeling," he added.
"We have seen with Simone Biles you can speak on how you are feeling and hopefully feel supported by many people."