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."