The 25-year-old moved to the Stadium of Light from Manchester City last summer for £10m but he has failed to find consistent form and his international prospects have suffered as a result.
Injury kept the former Middlesbrough player out of the World Cup qualifiers against Moldova and Ukraine in September and he was an unused substitute for the matches with San Marino and Poland the following month.
He was overlooked for the friendly game with Sweden in November and last week's game with Brazil.
"It's always difficult because I have been in the squad for a while and then I have been overlooked for the last couple of games," Johnson told the Sunderland Echo.
"I have just got to get my head down and get better, really. If I do that, then I am confident that I can work my way back into the squad.
"But the key word, in terms of form, is consistency. I think I have only shown tiny glimpses of my best form over the five or six months that I have been here.
"It's difficult for all players - you know it isn't going to be rosy all the time - but you just have to come out the other end of it."
Johnson has made 28 appearances for Sunderland this season, scoring four goals.
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